Merge branch 'raimo/new-gen-state-machine/OTP-13065'

* raimo/new-gen-state-machine/OTP-13065: (52 commits)
  Add section on state filtering
  Promote gen_statem over gen_fsm
  Modify code_change/4 to return CallbackMode
  Use ?NAME macro in examples
  Introduce Fred Herbert suggested additions
  Introduce corrections from Fred Hebert and Ingela
  Use .png pictures instead of .gif
  Write Design Principles chapter
  Fix missing short forms for event timeout
  Do more intricate Fred Hebert doc changes
  Change Caller -> From as suggested by Fred Hebert
  Do documentation improvements from Fred Hebert
  Fix broken documenation reference
  Rename state_timeout -> event_timeout
  Fix most of the system docs and emacs mode
  Change code_change/4 to {ok,State,Data}
  Fixup sharpened test suite
  Sharpen test suite
  Remove the remove_event action and all alike
  Relax caller() type check and cleanup
  ...

Conflicts:
	lib/stdlib/src/gen.erl
	lib/stdlib/src/gen_event.erl
	lib/stdlib/src/gen_fsm.erl
	lib/stdlib/src/gen_server.erl
	lib/stdlib/test/error_logger_forwarder.erl

39 files changed, 6341 insertions, 244 deletions

diff --git a/lib/sasl/doc/src/appup.xml b/lib/sasl/doc/src/appup.xml
index a57068492c..6fbdcb9f5b 100644
--- a/lib/sasl/doc/src/appup.xml
+++ b/lib/sasl/doc/src/appup.xml
@@ -137,7 +137,8 @@
       code change. If it is set to <c>{advanced,Extra}</c>, implemented
       processes using 
       <seealso marker="stdlib:gen_server"><c>gen_server</c></seealso>, 
-      <seealso marker="stdlib:gen_fsm"><c>gen_fsm</c></seealso>, or
+      <seealso marker="stdlib:gen_fsm"><c>gen_fsm</c></seealso>,
+      <seealso marker="stdlib:gen_statem"><c>gen_statem</c></seealso>, or
       <seealso marker="stdlib:gen_event"><c>gen_event</c></seealso> 
       transform their internal state by calling the callback function
       <c>code_change</c>. Special processes call the callback
diff --git a/lib/sasl/doc/src/error_logging.xml b/lib/sasl/doc/src/error_logging.xml
index 86d0c811e9..8464a41ff9 100644
--- a/lib/sasl/doc/src/error_logging.xml
+++ b/lib/sasl/doc/src/error_logging.xml
@@ -90,8 +90,9 @@
       a process terminates with an unexpected reason, which is any reason 
       other than <c>normal</c>, <c>shutdown</c>, or <c>{shutdown,Term}</c>.
       Processes using behaviors
-      <seealso marker="stdlib:gen_server"><c>gen_server</c></seealso> or 
-      <seealso marker="stdlib:gen_fsm"><c>gen_fsm</c></seealso> 
+      <seealso marker="stdlib:gen_server"><c>gen_server</c></seealso>,
+      <seealso marker="stdlib:gen_fsm"><c>gen_fsm</c></seealso> or
+      <seealso marker="stdlib:gen_statem"><c>gen_statem</c></seealso>
       are examples of such processes. A crash report contains the following items:</p>
     <taglist>
       <tag><c>Crasher</c></tag>
diff --git a/lib/stdlib/doc/src/Makefile b/lib/stdlib/doc/src/Makefile
index 196c86748f..26602764a6 100644
--- a/lib/stdlib/doc/src/Makefile
+++ b/lib/stdlib/doc/src/Makefile
@@ -1,7 +1,7 @@
 #
 # %CopyrightBegin%
 #
-# Copyright Ericsson AB 1997-2015. All Rights Reserved.
+# Copyright Ericsson AB 1997-2016. All Rights Reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -68,6 +68,7 @@ XML_REF3_FILES = \
 	gen_event.xml \
 	gen_fsm.xml \
 	gen_server.xml \
+	gen_statem.xml \
 	io.xml \
 	io_lib.xml \
 	lib.xml \
diff --git a/lib/stdlib/doc/src/gen_fsm.xml b/lib/stdlib/doc/src/gen_fsm.xml
index 37e7c6ed17..835e252704 100644
--- a/lib/stdlib/doc/src/gen_fsm.xml
+++ b/lib/stdlib/doc/src/gen_fsm.xml
@@ -31,12 +31,23 @@
   <module>gen_fsm</module>
   <modulesummary>Generic Finite State Machine Behaviour</modulesummary>
   <description>
+    <note>
+      <p>
+	There is a new behaviour
+	<seealso marker="gen_statem"><c>gen_statem</c></seealso>
+	that is intended to replace <c>gen_fsm</c> for new code.
+	It has the same features and add some really useful.
+	This module will not be removed for the foreseeable future
+	to keep old state machine implementations running.
+      </p>
+    </note>
     <p>A behaviour module for implementing a finite state machine.
       A generic finite state machine process (gen_fsm) implemented
       using this module will have a standard set of interface functions
       and include functionality for tracing and error reporting. It will
       also fit into an OTP supervision tree. Refer to
-      <seealso marker="doc/design_principles:fsm">OTP Design Principles</seealso> for more information.</p>
+      <seealso marker="doc/design_principles:fsm">OTP Design Principles</seealso> for more information.
+    </p>
     <p>A gen_fsm assumes all specific parts to be located in a callback
       module exporting a pre-defined set of functions. The relationship
       between the behaviour functions and the callback functions can be
@@ -848,6 +859,7 @@ gen_fsm:sync_send_all_state_event -----> Module:handle_sync_event/4
     <title>SEE ALSO</title>
     <p><seealso marker="gen_event">gen_event(3)</seealso>,
       <seealso marker="gen_server">gen_server(3)</seealso>,
+      <seealso marker="gen_statem">gen_statem(3)</seealso>,
       <seealso marker="supervisor">supervisor(3)</seealso>,
       <seealso marker="proc_lib">proc_lib(3)</seealso>,
       <seealso marker="sys">sys(3)</seealso></p>
diff --git a/lib/stdlib/doc/src/gen_server.xml b/lib/stdlib/doc/src/gen_server.xml
index 02da11250a..10dc978afc 100644
--- a/lib/stdlib/doc/src/gen_server.xml
+++ b/lib/stdlib/doc/src/gen_server.xml
@@ -715,6 +715,7 @@ gen_server:abcast     -----> Module:handle_cast/2
     <title>SEE ALSO</title>
     <p><seealso marker="gen_event">gen_event(3)</seealso>,
       <seealso marker="gen_fsm">gen_fsm(3)</seealso>,
+      <seealso marker="gen_statem">gen_statem(3)</seealso>,
       <seealso marker="supervisor">supervisor(3)</seealso>,
       <seealso marker="proc_lib">proc_lib(3)</seealso>,
       <seealso marker="sys">sys(3)</seealso></p>
diff --git a/lib/stdlib/doc/src/gen_statem.xml b/lib/stdlib/doc/src/gen_statem.xml
new file mode 100644
index 0000000000..91332fbdde
--- /dev/null
+++ b/lib/stdlib/doc/src/gen_statem.xml
@@ -0,0 +1,1715 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<!DOCTYPE erlref SYSTEM "erlref.dtd">
+
+<erlref>
+  <header>
+    <copyright>
+      <year>2016</year>
+      <holder>Ericsson AB. All Rights Reserved.</holder>
+    </copyright>
+    <legalnotice>
+      Licensed under the Apache License, Version 2.0 (the "License");
+      you may not use this file except in compliance with the License.
+      You may obtain a copy of the License at
+
+          http://www.apache.org/licenses/LICENSE-2.0
+
+      Unless required by applicable law or agreed to in writing, software
+      distributed under the License is distributed on an "AS IS" BASIS,
+      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+      See the License for the specific language governing permissions and
+      limitations under the License.
+
+    </legalnotice>
+
+    <title>gen_statem</title>
+    <prepared></prepared>
+    <docno></docno>
+    <date></date>
+    <rev></rev>
+  </header>
+  <module>gen_statem</module>
+  <modulesummary>Generic State Machine Behaviour</modulesummary>
+  <description>
+    <p>
+      A behaviour module for implementing a state machine. Two
+      <seealso marker="#type-callback_mode"><em>callback modes</em></seealso>
+      are supported.  One for finite state machines
+      (<seealso marker="gen_fsm"><c>gen_fsm</c></seealso> like)
+      that requires the state to be an atom and uses that state as
+      the name of the current callback function,
+      and one without restriction on the state data type
+      that uses one callback function for all states.
+    </p>
+    <p>
+      This is a new behaviour in OTP-19.0.
+      It has been thoroughly reviewed, is stable enough
+      to be used by at least two heavy OTP applications, and is here to stay.
+      But depending on user feedback, we do not expect
+      but might find it necessary to make minor
+      not backwards compatible changes into OTP-20.0,
+      so its state can be designated as "not quite experimental"...
+    </p>
+    <p>
+      The <c>gen_statem</c> behaviour is intended to replace
+      <seealso marker="gen_fsm"><c>gen_fsm</c></seealso> for new code.
+      It has the same features and add some really useful:
+    </p>
+    <list type="bulleted">
+      <item>State code is gathered</item>
+      <item>The state can be any term</item>
+      <item>Events can be postponed</item>
+      <item>Events can be self generated</item>
+      <item>A reply can be sent from a later state</item>
+      <item>There can be multiple sys traceable replies</item>
+    </list>
+    <p>
+      The callback model(s) for <c>gen_statem</c> differs from
+      the one for <seealso marker="gen_fsm"><c>gen_fsm</c></seealso>,
+      but it is still fairly easy to rewrite
+      from <c>gen_fsm</c> to <c>gen_statem</c>.
+    </p>
+    <p>
+      A generic state machine process (<c>gen_statem</c>) implemented
+      using this module will have a standard set of interface functions
+      and include functionality for tracing and error reporting.
+      It will also fit into an OTP supervision tree. Refer to
+      <seealso marker="doc/design_principles:statem">
+        OTP Design Principles
+      </seealso>
+      for more information.
+    </p>
+    <p>
+      A <c>gen_statem</c> assumes all specific parts to be located in a
+      callback module exporting a pre-defined set of functions.
+      The relationship between the behaviour functions and the callback
+      functions can be illustrated as follows:</p>
+    <pre>
+gen_statem module            Callback module
+-----------------            ---------------
+gen_statem:start
+gen_statem:start_link -----> Module:init/1
+
+gen_statem:stop       -----> Module:terminate/3
+
+gen_statem:call
+gen_statem:cast
+erlang:send
+erlang:'!'            -----> Module:StateName/3
+                             Module:handle_event/4
+
+-                     -----> Module:terminate/3
+
+-                     -----> Module:code_change/4</pre>
+    <p>
+      Events are of different
+      <seealso marker="#type-event_type">types</seealso>
+      so the callback functions can know the origin of an event
+      and how to respond.
+    </p>
+    <p>
+      If a callback function fails or returns a bad value,
+      the <c>gen_statem</c> will terminate.  An exception of class
+      <seealso marker="erts:erlang#throw/1"><c>throw</c></seealso>,
+      however, is not regarded as an error but as a valid return.
+    </p>
+    <marker id="state_function" />
+    <p>
+      The "<em>state function</em>" for a specific
+      <seealso marker="#type-state">state</seealso>
+      in a <c>gen_statem</c> is the callback function that is called
+      for all events in this state, and is selected depending on which
+      <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
+      that the implementation specifies when the the server starts.
+    </p>
+    <p>
+      When the
+      <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
+      is <c>state_functions</c>, the state has to be an atom and
+      is used as the state function name.  See
+      <seealso marker="#Module:StateName/3">
+        <c>Module:StateName/3</c>
+      </seealso>.
+      This gathers all code for a specific state
+      in one function and hence dispatches on state first.
+      Note that in this mode the fact that there is
+      a mandatory callback function
+      <seealso marker="#Module:terminate/3">
+        <c>Module:terminate/3</c>
+      </seealso> makes the state name <c>terminate</c> unusable.
+    </p>
+    <p>
+      When the
+      <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
+      is <c>handle_event_function</c> the state can be any term
+      and the state function name is
+      <seealso marker="#Module:handle_event/4">
+        <c>Module:handle_event/4</c>
+      </seealso>.
+      This makes it easy to dispatch on state or on event as you desire.
+      Be careful about which events you handle in which
+      states so you do not accidentally postpone one event
+      forever creating an infinite busy loop.
+    </p>
+    <p>
+      The <c>gen_statem</c> enqueues incoming events in order of arrival
+      and presents these to the
+      <seealso marker="#state_function">state function</seealso>
+      in that order.  The state function can postpone an event
+      so it is not retried in the current state.
+      After a state change the queue restarts with the postponed events.
+    </p>
+    <p>
+      The <c>gen_statem</c> event queue model is sufficient
+      to emulate the normal process message queue with selective receive.
+      Postponing an event corresponds to not matching it
+      in a receive statement and changing states corresponds
+      to entering a new receive statement.
+    </p>
+    <p>
+      The <seealso marker="#state_function">state function</seealso>
+      can insert events using the
+      <seealso marker="#type-action">
+	<c>action()</c> <c>next_event</c>
+      </seealso>
+      and such an event is inserted as the next to present
+      to the state function.  That is: as if it is
+      the oldest incoming event.  There is a dedicated
+      <seealso marker="#type-event_type">
+        <c>event_type()</c>
+      </seealso>
+      <c>internal</c> that can be used for such events making them impossible
+      to mistake for external events.
+    </p>
+    <p>
+      Inserting an event replaces the trick of calling your own
+      state handling functions that you often would have to
+      resort to in for example <seealso marker="gen_fsm"><c>gen_fsm</c></seealso>
+      to force processing an inserted event before others.
+      A warning, though: if you in <c>gen_statem</c> for example
+      postpone an event in one state and then call some other state function of yours,
+      you have not changed states and hence the postponed event will not be retried,
+      which is logical but might be confusing.
+    </p>
+    <p>
+      See the type
+      <seealso marker="#type-transition_option">
+	<c>transition_option()</c>
+      </seealso>
+      for the details of a state transition.
+    </p>
+    <p>
+      A <c>gen_statem</c> handles system messages as documented in
+      <seealso marker="sys"><c>sys</c></seealso>.
+      The <c>sys</c>module can be used for debugging a <c>gen_statem</c>.
+    </p>
+    <p>
+      Note that a <c>gen_statem</c> does not trap exit signals
+      automatically, this must be explicitly initiated in
+      the callback module (by calling
+      <seealso marker="erts:erlang#process_flag/2">
+	<c>process_flag(trap_exit, true)</c></seealso>.
+    </p>
+    <p>
+      Unless otherwise stated, all functions in this module fail if
+      the specified <c>gen_statem</c> does not exist or
+      if bad arguments are given.
+    </p>
+    <p>
+      The <c>gen_statem</c> process can go into hibernation (see
+      <seealso marker="erts:erlang#hibernate/3">
+	<c>erlang:hibernate/3</c>
+      </seealso>) if a
+      <seealso marker="#state_function">state function</seealso> or
+      <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
+      specifies <c>hibernate</c> in the returned
+      <seealso marker="#type-action"><c>Actions</c></seealso>
+      list. This might be useful if the server is expected to be idle
+      for a long time. However use this feature with care
+      since hibernation implies at least two garbage collections
+      (when hibernating and shortly after waking up) and that is not
+      something you would want to do between each event on a busy server.
+    </p>
+  </description>
+
+  <section>
+    <title>EXAMPLE</title>
+    <p>
+      This example shows a simple pushbutton model
+      for a toggling pushbutton implemented with
+      <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
+      <c>state_functions</c>.
+      You can push the button and it replies if it went on or off,
+      and you can ask for a count of how many times it has been
+      pushed to on.
+    </p>
+    <p>This is the complete callback module file <c>pushbutton.erl</c>:</p>
+    <code type="erl">
+-module(pushbutton).
+-behaviour(gen_statem).
+
+-export([start/0,push/0,get_count/0,stop/0]).
+-export([terminate/3,code_change/4,init/1]).
+-export([on/3,off/3]).
+
+name() -> pushbutton_statem. % The registered server name
+callback_mode() -> state_functions.
+
+%% API.  This example uses a registered name name()
+%% and does not link to the caller.
+start() ->
+    gen_statem:start({local,name()}, ?MODULE, [], []).
+push() ->
+    gen_statem:call(name(), push).
+get_count() ->
+    gen_statem:call(name(), get_count).
+stop() ->
+    gen_statem:stop(name()).
+
+%% Mandatory callback functions
+terminate(_Reason, _State, _Data) ->
+    void.
+code_change(_Vsn, State, Data, _Extra) ->
+    {callback_mode(),State,Data}.
+init([]) ->
+    %% Set the callback mode and initial state + data.
+    %% Data is used only as a counter.
+    State = off, Data = 0,
+    {callback_mode(),State,Data}.
+
+
+%%% State functions
+
+off({call,From}, push, Data) ->
+    %% Go to 'on', increment count and reply
+    %% that the resulting status is 'on'
+    {next_state,on,Data+1,[{reply,From,on}]};
+off(EventType, EventContent, Data) ->
+    handle_event(EventType, EventContent, Data).
+
+on({call,From}, push, Data) ->
+    %% Go to 'off' and reply that the resulting status is 'off'
+    {next_state,off,Data,[{reply,From,off}]};
+on(EventType, EventContent, Data) ->
+    handle_event(EventType, EventContent, Data).
+
+%% Handle events common to all states
+handle_event({call,From}, get_count, Data) ->
+    %% Reply with the current count
+    {keep_state,Data,[{reply,From,Data}]};
+handle_event(_, _, Data) ->
+    %% Ignore all other events
+    {keep_state,Data}.
+    </code>
+    <p>And this is a shell session when running it:</p>
+    <pre>
+1> pushbutton:start().
+{ok,&lt;0.36.0>}
+2> pushbutton:get_count().
+0
+3> pushbutton:push().
+on
+4> pushbutton:get_count().
+1
+5> pushbutton:push().
+off
+6> pushbutton:get_count().
+1
+7> pushbutton:stop().
+ok
+8> pushbutton:push().
+** exception exit: {noproc,{gen_statem,call,[pushbutton_statem,push,infinity]}}
+     in function  gen:do_for_proc/2 (gen.erl, line 261)
+     in call from gen_statem:call/3 (gen_statem.erl, line 386)
+    </pre>
+
+    <p>
+      And just to compare styles here is the same example using
+      <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
+      <c>state_functions</c>, or rather here is code to replace
+      from the <c>init/1</c> function of the <c>pushbutton.erl</c>
+      example file above:
+    </p>
+    <code type="erl">
+init([]) ->
+    %% Set the callback mode and initial state + data.
+    %% Data is used only as a counter.
+    State = off, Data = 0,
+    {handle_event_function,State,Data}.
+
+
+%%% Event handling
+
+handle_event({call,From}, push, off, Data) ->
+    %% Go to 'on', increment count and reply
+    %% that the resulting status is 'on'
+    {next_state,on,Data+1,[{reply,From,on}]};
+handle_event({call,From}, push, on, Data) ->
+    %% Go to 'off' and reply that the resulting status is 'off'
+    {next_state,off,Data,[{reply,From,off}]};
+%%
+%% Event handling common to all states
+handle_event({call,From}, get_count, State, Data) ->
+    %% Reply with the current count
+    {next_state,State,Data,[{reply,From,Data}]};
+handle_event(_, _, State, Data) ->
+    %% Ignore all other events
+    {next_state,State,Data}.
+    </code>
+  </section>
+
+  <datatypes>
+    <datatype>
+      <name name="server_name" />
+      <desc>
+	<p>
+	  Name specification to use when starting
+	  a <c>gen_statem</c> server.  See
+	  <seealso marker="#start_link/3">
+	    <c>start_link/3</c>
+	  </seealso>
+	  and
+	  <seealso marker="#type-server_ref">
+	    <c>server_ref()</c>
+	  </seealso> below.
+	</p>
+      </desc>
+    </datatype>
+    <datatype>
+      <name name="server_ref" />
+      <desc>
+	<p>
+	  Server specification to use when addressing
+	  a <c>gen_statem</c> server.
+	  See <seealso marker="#call/2"><c>call/2</c></seealso> and
+	  <seealso marker="#type-server_name">
+	    <c>server_name()</c>
+	  </seealso>
+	  above.
+	</p>
+	<p>It can be:</p>
+	<taglist>
+	  <tag><c>pid()</c><br />
+	  <c>LocalName</c></tag>
+	  <item>The <c>gen_statem</c> is locally registered.</item>
+	  <tag><c>Name, Node</c></tag>
+	  <item>
+	    The <c>gen_statem</c> is locally registered
+	    on another node.
+	  </item>
+	  <tag><c>GlobalName</c></tag>
+	  <item>
+	    The <c>gen_statem</c> is globally registered
+	    in <seealso marker="kernel:global"><c>global</c></seealso>.
+	  </item>
+	  <tag><c>RegMod, ViaName</c></tag>
+	  <item>
+	    The <c>gen_statem</c> is registered through
+	    an alternative process registry.
+	    The registry callback module <c>RegMod</c>
+	    should export the functions
+	    <c>register_name/2</c>, <c>unregister_name/1</c>,
+	    <c>whereis_name/1</c> and <c>send/2</c>,
+	    which should behave like the corresponding functions
+	    in <seealso marker="kernel:global"><c>global</c></seealso>.
+	    Thus, <c>{via,global,GlobalName}</c> is the same as
+	    <c>{global,GlobalName}</c>.
+	  </item>
+	</taglist>
+      </desc>
+    </datatype>
+    <datatype>
+      <name name="debug_opt" />
+      <desc>
+	<p>
+	  Debug option that can be used when starting
+	  a <c>gen_statem</c> server through for example
+	  <seealso marker="#enter_loop/5"><c>enter_loop/5</c></seealso>.
+	</p>
+	<p>
+	  For every entry in <c><anno>Dbgs</anno></c>
+	  the corresponding function in
+	  <seealso marker="sys"><c>sys</c></seealso> will be called.
+	</p>
+      </desc>
+    </datatype>
+    <datatype>
+      <name name="start_opt" />
+      <desc>
+	<p>
+	  Options that can be used when starting
+	  a <c>gen_statem</c> server through for example
+	  <seealso marker="#start_link/3"><c>start_link/3</c></seealso>.
+	</p>
+      </desc>
+    </datatype>
+    <datatype>
+      <name name="start_ret" />
+      <desc>
+	<p>
+	  Return value from the start functions for_example
+	  <seealso marker="#start_link/3"><c>start_link/3</c></seealso>.
+	</p>
+      </desc>
+    </datatype>
+
+    <datatype>
+      <name name="from" />
+      <desc>
+	<p>
+	  Destination to use when replying through for example the
+	  <seealso marker="#type-action">
+	    <c>action()</c>
+	  </seealso>
+	  <c>{reply,From,Reply}</c>
+	  to a process that has called the <c>gen_statem</c> server using
+	  <seealso marker="#call/2"><c>call/2</c></seealso>.
+	</p>
+      </desc>
+    </datatype>
+    <datatype>
+      <name name="state" />
+      <desc>
+	<p>
+	  After a state change (<c>NextState =/= State</c>)
+	  all postponed events are retried.
+	</p>
+      </desc>
+    </datatype>
+    <datatype>
+      <name name="state_name" />
+      <desc>
+	<p>
+	  If the
+	  <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
+	  is <c>state_functions</c>,
+	  the state has to be of this type.
+	</p>
+      </desc>
+    </datatype>
+    <datatype>
+      <name name="data" />
+      <desc>
+	<p>
+	  A term in which the state machine implementation
+	  should store any server data it needs.  The difference between
+	  this and the <seealso marker="#type-state"><c>state()</c></seealso>
+	  itself is that a change in this data does not cause
+	  postponed events to be retried.  Hence if a change
+	  in this data would change the set of events that
+	  are handled than that data item should be made
+	  a part of the state.
+	</p>
+      </desc>
+    </datatype>
+    <datatype>
+      <name name="event_type" />
+      <desc>
+	<p>
+	  External events are of 3 different type:
+	  <c>{call,<anno>From</anno>}</c>, <c>cast</c> or <c>info</c>.
+	  <seealso marker="#call/2">Calls</seealso>
+	  (synchronous) and
+	  <seealso marker="#cast/2">casts</seealso>
+	  originate from the corresponding API functions.
+	  For calls the event contain whom to reply to.
+	  Type <c>info</c> originates from regular process messages sent
+	  to the <c>gen_statem</c>. It is also possible for the state machine
+	  implementation to generate events of types
+	  <c>timeout</c> and <c>internal</c> to itself.
+	</p>
+      </desc>
+    </datatype>
+    <datatype>
+      <name name="callback_mode" />
+      <desc>
+	<p>
+	  The <em>callback mode</em> is selected when starting the
+	  <c>gen_statem</c> using the return value from
+	  <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
+	  or when calling
+	  <seealso marker="#enter_loop/5"><c>enter_loop/5-7</c></seealso>,
+	  and with the return value from
+	  <seealso marker="#Module:code_change/4">
+	    <c>Module:code_change/4</c>.
+	  </seealso>
+	</p>
+	<taglist>
+	  <tag><c>state_functions</c></tag>
+	  <item>
+	    The state has to be of type
+	    <seealso marker="#type-state_name"><c>state_name()</c></seealso>
+	    and one callback function per state that is
+	    <seealso marker="#Module:StateName/3">
+	      <c>Module:StateName/3</c>
+	    </seealso>
+	    is used.
+	  </item>
+	  <tag><c>handle_event_function</c></tag>
+	  <item>
+	    The state can be any term and the callback function
+	    <seealso marker="#Module:handle_event/4">
+	      <c>Module:handle_event/4</c>
+	    </seealso>
+	    is used for all states.
+	  </item>
+	</taglist>
+      </desc>
+    </datatype>
+    <datatype>
+      <name name="transition_option" />
+      <desc>
+	<p>
+	  Transition options may be set by
+	  <seealso marker="#type-action">actions</seealso>
+	  and they modify some details below in how
+	  the state transition is done:
+	</p>
+	<list type="ordered">
+	  <item>
+	    All
+            <seealso marker="#type-action">actions</seealso>
+	    are processed in order of appearance.
+	  </item>
+	  <item>
+	    If
+            <seealso marker="#type-postpone">
+	      <c>postpone()</c>
+	    </seealso>
+	    is <c>true</c>
+	    the current event is postponed.
+	  </item>
+	  <item>
+	    If the state changes the queue of incoming events
+            is reset to start with the oldest postponed.
+	  </item>
+	  <item>
+	    All events stored with
+	    <seealso marker="#type-action">
+	      <c>action()</c>
+	    </seealso>
+	    <c>next_event</c>
+	    are inserted in the queue to be processed before
+	    all other events.
+	  </item>
+	  <item>
+	    If an
+            <seealso marker="#type-event_timeout">
+	      <c>event_timeout()</c>
+	    </seealso>
+	    is set through
+            <seealso marker="#type-action">
+	      <c>action()</c>
+	    </seealso>
+	    <c>timeout</c>
+	    an event timer may be started or a timeout zero event
+	    may be enqueued.
+	  </item>
+	  <item>
+	    The (possibly new)
+            <seealso marker="#state_function">state function</seealso>
+            is called with the oldest enqueued event if there is any,
+	    otherwise the <c>gen_statem</c> goes into <c>receive</c>
+	    or hibernation
+	    (if
+            <seealso marker="#type-hibernate">
+	      <c>hibernate()</c>
+	    </seealso>
+	    is <c>true</c>)
+	    to wait for the next message.  In hibernation the next
+	    non-system event awakens the <c>gen_statem</c>, or rather
+	    the next incoming message awakens the <c>gen_statem</c>
+	    but if it is a system event it goes back into hibernation.
+	  </item>
+	</list>
+      </desc>
+    </datatype>
+    <datatype>
+      <name name="postpone" />
+      <desc>
+	<p>
+	  If <c>true</c> postpone the current event and retry
+	  it when the state changes
+	  (<c>NextState =/= State</c>).
+	</p>
+      </desc>
+    </datatype>
+    <datatype>
+      <name name="hibernate" />
+      <desc>
+	<p>
+	  If <c>true</c> hibernate the <c>gen_statem</c>
+	  by calling
+	  <seealso marker="proc_lib#hibernate/3">
+	    <c>proc_lib:hibernate/3</c>
+	  </seealso>
+	  before going into <c>receive</c>
+	  to wait for a new external event.
+	  If there are enqueued events the <c>hibernate</c>
+	  is ignored as if an event just arrived and awakened
+	  the <c>gen_statem</c>.
+	</p>
+      </desc>
+    </datatype>
+    <datatype>
+      <name name="event_timeout" />
+      <desc>
+	<p>
+	  Generate an event of
+	  <seealso marker="#type-event_type"><c>event_type()</c></seealso>
+	  <c>timeout</c>
+	  after this time (in milliseconds) unless some other
+	  event arrives in which case this timeout is cancelled.
+	  Note that a retried or inserted event
+	  counts just like a new in this respect.
+	</p>
+	<p>
+	  If the value is <c>infinity</c> no timer is started since
+	  it will never trigger anyway.
+	</p>
+	<p>
+	  If the value is <c>0</c> the timeout event is immediately enqueued
+	  unless there already are enqueued events since then the
+	  timeout is immediately cancelled.
+	  This is a feature ensuring that a timeout <c>0</c> event
+	  will be processed before any not yet received external event.
+	</p>
+	<p>
+	  Note that it is not possible nor needed to cancel this timeout
+	  since it is cancelled automatically by any other event.
+	</p>
+      </desc>
+    </datatype>
+    <datatype>
+      <name name="action" />
+      <desc>
+	<p>
+	  These state transition actions may be invoked by
+	  returning them from the
+	  <seealso marker="#state_function">state function</seealso>,
+	  from <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
+	  or by giving them to
+	  <seealso marker="#enter_loop/6"><c>enter_loop/6,7</c></seealso>.
+	</p>
+	<p>
+	  Actions are executed in the containing list order.
+	</p>
+	<p>
+	  Actions that set
+	  <seealso marker="#type-transition_option">
+	    transition options
+	  </seealso>
+	  overrides any previous of the same type,
+	  so the last in the containing list wins.
+	  For example the last
+	  <seealso marker="#type-event_timeout">
+	    <c>event_timeout()</c>
+	  </seealso>
+	  overrides any other <c>event_timeout()</c> in the list.
+	</p>
+	<taglist>
+	  <tag><c>postpone</c></tag>
+	  <item>
+	    Set the
+	    <seealso marker="#type-transition_option">
+	      <c>transition_option()</c>
+	    </seealso>
+	    <seealso marker="#type-postpone">
+	      <c>postpone()</c>
+	    </seealso>
+	    for this state transition.
+	    This action is ignored when returned from
+	    <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
+	    or given to
+	    <seealso marker="#enter_loop/5"><c>enter_loop/5,6</c></seealso>
+	    since there is no event to postpone in those cases.
+	  </item>
+	  <tag><c>hibernate</c></tag>
+	  <item>
+	    Set the
+	    <seealso marker="#type-transition_option">
+	      <c>transition_option()</c>
+	    </seealso>
+	    <seealso marker="#type-hibernate">
+	      <c>hibernate()</c>
+	    </seealso>
+	    for this state transition.
+	  </item>
+	  <tag><c>Timeout</c></tag>
+	  <item>
+	    Short for <c>{timeout,Timeout,Timeout}</c> that is
+	    the timeout message is the timeout time.
+	    This form exists to make the
+	    <seealso marker="#state_function">state function</seealso>
+	    return value <c>{next_state,NextState,NewData,Timeout}</c>
+	    allowed like for
+	    <seealso marker="gen_fsm#Module:StateName/2">
+	      <c>gen_fsm Module:StateName/2</c>.
+	    </seealso>
+	  </item>
+	  <tag><c>timeout</c></tag>
+	  <item>
+	    Set the
+	    <seealso marker="#type-transition_option">
+	      <c>transition_option()</c>
+	    </seealso>
+	    <seealso marker="#type-event_timeout">
+	      <c>event_timeout()</c>
+	    </seealso>
+	    to <c><anno>Time</anno></c> with <c><anno>EventContent</anno></c>.
+	  </item>
+	  <tag><c>reply_action()</c></tag>
+	  <item>Reply to a caller.</item>
+	  <tag><c>next_event</c></tag>
+	  <item>
+	      Store the given <c><anno>EventType</anno></c>
+	      and <c><anno>EventContent</anno></c> for insertion after all
+	      actions have been executed.
+	  </item>
+	  <item>
+	    <p>
+	      The stored events are inserted in the queue as the next to process
+	      before any already queued events.  The order of these stored events
+	      is preserved so the first <c>next_event</c> in the containing list
+	      will become the first to process.
+	    </p>
+	  </item>
+	  <item>
+	    <p>
+	      An event of type
+	      <seealso marker="#type-event_type">
+		<c>internal</c>
+	      </seealso>
+	      should be used when you want to reliably distinguish
+	      an event inserted this way from any external event.
+	    </p>
+	  </item>
+	</taglist>
+      </desc>
+    </datatype>
+    <datatype>
+      <name name="reply_action" />
+      <desc>
+	<p>
+	  Reply to a caller waiting for a reply in
+	  <seealso marker="#call/2"><c>call/2</c></seealso>.
+          <c><anno>From</anno></c> must be the term from the
+	  <seealso marker="#type-event_type">
+	    <c>{call,<anno>From</anno>}</c>
+	  </seealso>
+	  argument to the
+	  <seealso marker="#state_function">state function</seealso>.
+	</p>
+      </desc>
+    </datatype>
+    <datatype>
+      <name name="state_function_result" />
+      <desc>
+	<taglist>
+	  <tag><c>next_state</c></tag>
+	  <item>
+	    The <c>gen_statem</c> will do a state transition to
+	    <c><anno>NextStateName</anno></c>
+	    (which may be the same as the current state),
+	    set <c><anno>NewData</anno></c>
+	    and execute all <c><anno>Actions</anno></c>
+	  </item>
+	</taglist>
+	<p>
+	  All these terms are tuples or atoms and this property
+	  will hold in any future version of <c>gen_statem</c>,
+	  just in case you need such a promise.
+	</p>
+      </desc>
+    </datatype>
+    <datatype>
+      <name name="handle_event_result" />
+      <desc>
+	<taglist>
+	  <tag><c>next_state</c></tag>
+	  <item>
+	    The <c>gen_statem</c> will do a state transition to
+	    <c><anno>NextState</anno></c>
+	    (which may be the same as the current state),
+	    set <c><anno>NewData</anno></c>
+	    and execute all <c><anno>Actions</anno></c>
+	  </item>
+	</taglist>
+	<p>
+	  All these terms are tuples or atoms and this property
+	  will hold in any future version of <c>gen_statem</c>,
+	  just in case you need such a promise.
+	</p>
+      </desc>
+    </datatype>
+    <datatype>
+      <name name="common_state_callback_result" />
+      <desc>
+	<taglist>
+	  <tag><c>stop</c></tag>
+	  <item>
+	    Terminate the <c>gen_statem</c> by calling
+	    <seealso marker="#Module:terminate/3">
+	      <c>Module:terminate/3</c>
+	    </seealso>
+	    with <c>Reason</c> and
+	    <c><anno>NewData</anno></c>, if given.
+	  </item>
+	  <tag><c>stop_and_reply</c></tag>
+	  <item>
+	    Send all <c><anno>Replies</anno></c>
+	    then terminate the <c>gen_statem</c> by calling
+	    <seealso marker="#Module:terminate/3">
+	      <c>Module:terminate/3</c>
+	    </seealso>
+	    with <c>Reason</c> and
+	    <c><anno>NewData</anno></c>, if given.
+	  </item>
+	  <tag><c>keep_state</c></tag>
+	  <item>
+	    The <c>gen_statem</c> will keep the current state, or
+	    do a state transition to the current state if you like,
+	    set <c><anno>NewData</anno></c>
+	    and execute all <c><anno>Actions</anno></c>.
+	    This is the same as
+	    <c>{next_state,CurrentState,<anno>NewData</anno>,<anno>Actions</anno>}</c>.
+	  </item>
+	  <tag><c>keep_state_and_data</c></tag>
+	  <item>
+	    The <c>gen_statem</c> will keep the current state or
+	    do a state transition to the current state if you like,
+	    keep the current server data,
+	    and execute all <c><anno>Actions</anno></c>.
+	    This is the same as
+	    <c>{next_state,CurrentState,CurrentData,<anno>Actions</anno>}</c>.
+	  </item>
+	</taglist>
+	<p>
+	  All these terms are tuples or atoms and this property
+	  will hold in any future version of <c>gen_statem</c>,
+	  just in case you need such a promise.
+	</p>
+      </desc>
+    </datatype>
+  </datatypes>
+
+  <funcs>
+
+    <func>
+      <name name="start_link" arity="3" />
+      <name name="start_link" arity="4" />
+      <fsummary>Create a linked <c>gen_statem</c> process</fsummary>
+      <desc>
+        <p>
+	  Creates a <c>gen_statem</c> process according
+	  to OTP design principles
+	  (using
+          <seealso marker="proc_lib"><c>proc_lib</c></seealso>
+	  primitives)
+	  that is linked to the calling process.
+	  This is essential when the <c>gen_statem</c> shall be part of
+	  a supervision tree so it gets linked to its supervisor.
+	</p>
+        <p>
+	  The <c>gen_statem</c> process calls
+	  <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
+	  to initialize the server. To ensure a synchronized start-up
+	  procedure, <c>start_link/3,4</c> does not return until
+	  <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
+          has returned.
+	</p>
+	<p>
+	  <c><anno>ServerName</anno></c> specifies the
+	  <seealso marker="#type-server_name">
+	    <c>server_name()</c>
+	  </seealso>
+	  to register for the <c>gen_statem</c>.
+	  If the <c>gen_statem</c> is started with  <c>start_link/3</c>
+	  no <c><anno>ServerName</anno></c> is provided and
+	  the <c>gen_statem</c> is not registered.
+	</p>
+        <p><c><anno>Module</anno></c> is the name of the callback module.</p>
+        <p>
+	  <c><anno>Args</anno></c> is an arbitrary term which is passed as
+          the argument to
+	  <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>.
+	</p>
+        <p>
+	  If the option <c>{timeout,Time}</c> is present in
+	  <c><anno>Opts</anno></c>, the <c>gen_statem</c>
+	  is allowed to spend <c>Time</c> milliseconds initializing
+	  or it will be terminated and the start function will return
+	  <seealso marker="#type-start_ret"><c>{error,timeout}</c></seealso>.
+	</p>
+        <p>
+	  If the option
+	  <seealso marker="#type-debug_opt"><c>{debug,Dbgs}</c></seealso>
+	  is present in <c><anno>Opts</anno></c>, debugging through
+	  <seealso marker="sys"><c>sys</c></seealso> is activated.
+	</p>
+        <p>
+	  If the option <c>{spawn_opt,SpawnOpts}</c> is present in
+	  <c><anno>Opts</anno></c>, <c>SpawnOpts</c> will be passed
+	  as option list to
+          <seealso marker="erts:erlang#spawn_opt/2"><c>spawn_opt/2</c></seealso>
+	  which is used to spawn the <c>gen_statem</c> process.
+	</p>
+        <note>
+          <p>
+	    Using the spawn option <c>monitor</c> is currently not
+            allowed, but will cause this function to fail with reason
+            <c>badarg</c>.
+	  </p>
+        </note>
+        <p>
+	  If the <c>gen_statem</c> is successfully created
+	  and initialized this function returns
+	  <seealso marker="#type-start_ret">
+	    <c>{ok,Pid}</c>,
+	  </seealso>
+	  where <c>Pid</c> is the <c>pid()</c>
+	  of the <c>gen_statem</c>.
+	  If there already exists a process with the specified
+	  <c><anno>ServerName</anno></c> this function returns
+	  <seealso marker="#type-start_ret"><c>{error,{already_started,Pid}}</c></seealso>,
+	  where <c>Pid</c> is the <c>pid()</c> of that process.
+	</p>
+        <p>
+	  If <c>Module:init/1</c> fails with <c>Reason</c>,
+          this function returns
+	  <seealso marker="#type-start_ret"><c>{error,Reason}</c></seealso>.
+	  If <c>Module:init/1</c> returns
+	  <seealso marker="#type-start_ret">
+	    <c>{stop,Reason}</c>
+	  </seealso>
+	  or
+	  <seealso marker="#type-start_ret"><c>ignore</c></seealso>,
+	  the process is terminated and this function
+          returns
+	  <seealso marker="#type-start_ret">
+	    <c>{error,Reason}</c>
+	  </seealso>
+	  or
+	  <seealso marker="#type-start_ret"><c>ignore</c></seealso>,
+	  respectively.
+	</p>
+      </desc>
+    </func>
+
+
+    <func>
+      <name name="start" arity="3" />
+      <name name="start" arity="4" />
+      <fsummary>Create a stand-alone <c>gen_statem</c> process</fsummary>
+      <desc>
+        <p>
+	  Creates a stand-alone <c>gen_statem</c> process according to
+	  OTP design principles (using
+          <seealso marker="proc_lib"><c>proc_lib</c></seealso>
+	  primitives).
+	  Since it does not get linked to the calling process
+	  this start function can not be used by a supervisor
+	  to start a child.
+	</p>
+        <p>
+	  See <seealso marker="#start_link/3"><c>start_link/3,4</c></seealso>
+          for a description of arguments and return values.
+	</p>
+      </desc>
+    </func>
+
+    <func>
+      <name name="stop" arity="1" />
+      <fsummary>Synchronously stop a generic server</fsummary>
+      <desc>
+	<p>
+	  The same as
+	  <seealso marker="#stop/3">
+	    <c>stop(<anno>ServerRef</anno>, normal, infinity)</c>.
+	  </seealso>
+	</p>
+      </desc>
+    </func>
+    <func>
+      <name name="stop" arity="3" />
+      <fsummary>Synchronously stop a generic server</fsummary>
+      <desc>
+	<p>
+	  Orders the <c>gen_statem</c>
+	  <seealso marker="#type-server_ref">
+	    <c><anno>ServerRef</anno></c>
+	  </seealso>
+	  to exit with the given <c><anno>Reason</anno></c>
+	  and waits for it to terminate.
+	  The <c>gen_statem</c> will call
+          <seealso marker="#Module:terminate/3">
+	    <c>Module:terminate/3</c>
+	  </seealso>
+	  before exiting.
+	</p>
+	<p>
+	  This function returns <c>ok</c> if the server terminates
+	  with the expected reason. Any other reason than <c>normal</c>,
+          <c>shutdown</c>, or <c>{shutdown,Term}</c> will cause an
+          error report to be issued through
+          <seealso marker="kernel:error_logger#format/2">
+	    <c>error_logger:format/2</c>.
+	  </seealso>
+	  The default <c><anno>Reason</anno></c> is <c>normal</c>.
+	</p>
+	<p>
+	  <c><anno>Timeout</anno></c> is an integer greater than zero
+	  which specifies how many milliseconds to wait for the server to
+          terminate, or the atom <c>infinity</c> to wait indefinitely.
+	  The default value is <c>infinity</c>.
+	  If the server has not terminated within the specified time,
+	  a <c>timeout</c> exception is raised.
+	</p>
+	<p>
+	  If the process does not exist, a <c>noproc</c> exception
+	  is raised.
+	</p>
+      </desc>
+    </func>
+
+    <func>
+      <name name="call" arity="2" />
+      <name name="call" arity="3" />
+      <fsummary>Make a synchronous call to a <c>gen_statem</c></fsummary>
+      <desc>
+        <p>
+	  Makes a synchronous call to the <c>gen_statem</c>
+	  <seealso marker="#type-server_ref">
+	    <c><anno>ServerRef</anno></c>
+	  </seealso>
+	  by sending a request
+	  and waiting until its reply arrives.
+	  The <c>gen_statem</c> will call the
+	  <seealso marker="#state_function">state function</seealso> with
+	  <seealso marker="#type-event_type"><c>event_type()</c></seealso>
+	  <c>{call,From}</c> and event content
+	  <c><anno>Request</anno></c>.
+	</p>
+	<p>
+	  A <c><anno>Reply</anno></c> is generated when a
+	  <seealso marker="#state_function">state function</seealso>
+	  returns with
+	  <c>{reply,From,<anno>Reply</anno>}</c> as one
+	  <seealso marker="#type-action"><c>action()</c></seealso>,
+	  and that <c><anno>Reply</anno></c> becomes the return value
+	  of this function.
+	</p>
+        <p>
+	  <c><anno>Timeout</anno></c> is an integer greater than zero
+	  which specifies how many milliseconds to wait for a reply,
+	  or the atom <c>infinity</c> to wait indefinitely,
+	  which is the default.  If no reply is received within
+	  the specified time, the function call fails.
+	</p>
+	<note>
+	  <p>
+	    To avoid getting a late reply in the caller's
+	    inbox this function spawns a proxy process that
+	    does the call.  A late reply gets delivered to the
+	    dead proxy process hence gets discarded.  This is
+	    less efficient than using
+	    <c><anno>Timeout</anno> =:= infinity</c>.
+	  </p>
+	</note>
+        <p>
+	  The call may fail for example if the <c>gen_statem</c> dies
+	  before or during this function call.
+	</p>
+      </desc>
+    </func>
+
+    <func>
+      <name name="cast" arity="2" />
+      <fsummary>Send an asynchronous event to a <c>gen_statem</c></fsummary>
+      <desc>
+	<p>
+	  Sends an asynchronous event to the <c>gen_statem</c>
+	  <seealso marker="#type-server_ref">
+	    <c><anno>ServerRef</anno></c>
+	  </seealso>
+	  and returns <c>ok</c> immediately,
+	  ignoring if the destination node or <c>gen_statem</c>
+	  does not exist.
+	  The <c>gen_statem</c> will call the
+	  <seealso marker="#state_function">state function</seealso> with
+	  <seealso marker="#type-event_type"><c>event_type()</c></seealso>
+	  <c>cast</c> and event content
+	  <c><anno>Msg</anno></c>.
+	</p>
+      </desc>
+    </func>
+
+    <func>
+      <name name="reply" arity="1" />
+      <name name="reply" arity="2" />
+      <fsummary>Reply to a caller</fsummary>
+      <desc>
+        <p>
+	  This function can be used by a <c>gen_statem</c>
+	  to explicitly send a reply to a process that waits in
+	  <seealso marker="#call/2"><c>call/2</c></seealso>
+          when the reply cannot be defined in
+          the return value of a
+	  <seealso marker="#state_function">state function</seealso>.
+	</p>
+        <p>
+	  <c><anno>From</anno></c> must be the term from the
+	  <seealso marker="#type-event_type">
+	    <c>{call,<anno>From</anno>}</c>
+	  </seealso>
+	  argument to the
+	  <seealso marker="#state_function">state function</seealso>.
+	  <c><anno>From</anno></c> and <c><anno>Reply</anno></c>
+	  can also be specified using a
+	  <seealso marker="#type-reply_action">
+	    <c>reply_action()</c>
+	  </seealso>
+	  and multiple replies with a list of them.
+	</p>
+	<note>
+	  <p>
+	    A reply sent with this function will not be visible
+	    in <seealso marker="sys"><c>sys</c></seealso> debug output.
+	  </p>
+	</note>
+      </desc>
+    </func>
+
+    <func>
+      <name name="enter_loop" arity="5" />
+      <fsummary>Enter the <c>gen_statem</c> receive loop</fsummary>
+      <desc>
+	<p>
+	  The same as
+	  <seealso marker="#enter_loop/7"><c>enter_loop/7</c></seealso>
+	  except that no
+	  <seealso marker="#type-server_name">
+	    <c>server_name()</c>
+	  </seealso>
+	  must have been registered.
+	</p>
+      </desc>
+    </func>
+    <func>
+      <name name="enter_loop" arity="6" />
+      <fsummary>Enter the <c>gen_statem</c> receive loop</fsummary>
+      <desc>
+	<p>
+	  If <c><anno>Server_or_Actions</anno></c> is a <c>list()</c>
+	  the same as
+	  <seealso marker="#enter_loop/7"><c>enter_loop/7</c></seealso>
+	  except that no
+	  <seealso marker="#type-server_name">
+	    <c>server_name()</c>
+	  </seealso>
+	  must have been registered and
+	  <c>Actions = <anno>Server_or_Actions</anno></c>.
+	</p>
+	<p>
+	  Otherwise the same as
+	  <seealso marker="#enter_loop/7"><c>enter_loop/7</c></seealso>
+	  with
+	  <c>Server = <anno>Server_or_Actions</anno></c> and
+	  <c>Actions = []</c>.
+	</p>
+      </desc>
+    </func>
+    <func>
+      <name name="enter_loop" arity="7" />
+      <fsummary>Enter the <c>gen_statem</c> receive loop</fsummary>
+      <desc>
+        <p>
+	  Makes an the calling process become a <c>gen_statem</c>.
+	  Does not return, instead the calling process will enter
+	  the <c>gen_statem</c> receive loop and become
+	  a <c>gen_statem</c> server.
+	  The process <em>must</em> have been started
+	  using one of the start functions in
+          <seealso marker="proc_lib"><c>proc_lib</c></seealso>.
+	  The user is responsible for any initialization of the process,
+	  including registering a name for it.
+	</p>
+        <p>
+	  This function is useful when a more complex initialization
+          procedure is needed than
+	  the <c>gen_statem</c> behaviour provides.
+	</p>
+        <p>
+	  <c><anno>Module</anno></c>, <c><anno>Opts</anno></c> and
+	  <c><anno>Server</anno></c> have the same meanings
+	  as when calling
+          <seealso marker="#start_link/3">
+	    <c>gen_statem:start[_link]/3,4</c>.
+	  </seealso>
+          However, the
+	  <seealso marker="#type-server_name">
+	    <c>server_name()</c>
+	  </seealso>
+	  name must have been registered accordingly
+	  <em>before</em> this function is called.</p>
+        <p>
+	  <c><anno>CallbackMode</anno></c>, <c><anno>State</anno></c>,
+	  <c><anno>Data</anno></c> and <c><anno>Actions</anno></c>
+	  have the same meanings as in the return value of
+          <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>.
+          Also, the callback module <c><anno>Module</anno></c>
+	  does not need to export an <c>init/1</c> function.
+	</p>
+        <p>
+	  Failure: If the calling process was not started by a
+          <seealso marker="proc_lib"><c>proc_lib</c></seealso>
+	  start function, or if it is not registered
+          according to
+	  <seealso marker="#type-server_name"><c>server_name()</c></seealso>.
+	</p>
+      </desc>
+    </func>
+
+  </funcs>
+
+
+
+  <section>
+    <title>CALLBACK FUNCTIONS</title>
+    <p>
+      The following functions should be exported from a
+      <c>gen_statem</c> callback module.
+    </p>
+  </section>
+  <funcs>
+
+    <func>
+      <name>Module:init(Args) -> Result</name>
+      <fsummary>Initialize process and internal state</fsummary>
+      <type>
+        <v>Args = term()</v>
+        <v>Result = {CallbackMode,State,Data}</v>
+	<v>&nbsp;| {CallbackMode,State,Data,Actions}</v>
+        <v>&nbsp;| {stop,Reason} | ignore</v>
+	<v>
+	  CallbackMode =
+	  <seealso marker="#type-callback_mode">callback_mode()</seealso>
+	</v>
+	<v>State = <seealso marker="#type-state">state()</seealso></v>
+	<v>
+	  Data = <seealso marker="#type-data">data()</seealso>
+	</v>
+	<v>
+	  Actions =
+	  [<seealso marker="#type-action">action()</seealso>] |
+	  <seealso marker="#type-action">action()</seealso>
+	</v>
+	<v>Reason = term()</v>
+      </type>
+      <desc>
+	<marker id="Module:init-1" />
+        <p>
+	  Whenever a <c>gen_statem</c> is started using
+          <seealso marker="#start_link/3"><c>start_link/3,4</c></seealso>
+	  or
+          <seealso marker="#start/3"><c>start/3,4</c></seealso>,
+          this function is called by the new process to initialize
+	  the implementation state and server data.
+	</p>
+        <p>
+	  <c>Args</c> is the <c>Args</c> argument provided to the start
+          function.
+	</p>
+        <p>
+	  If the initialization is successful, the function should
+          return <c>{CallbackMode,State,Data}</c> or
+	  <c>{CallbackMode,State,Data,Actions}</c>.
+	  <c>CallbackMode</c> selects the
+	  <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>.
+	  of the <c>gen_statem</c>.
+	  <c>State</c> is the initial
+	  <seealso marker="#type-state"><c>state()</c></seealso>
+	  and <c>Data</c> the initial server
+	  <seealso marker="#type-data"><c>data()</c></seealso>.
+	</p>
+	<p>
+	  The <seealso marker="#type-action"><c>Actions</c></seealso>
+	  are executed when entering the first
+	  <seealso marker="#type-state">state</seealso> just as for a
+	  <seealso marker="#state_function">state function</seealso>.
+	</p>
+        <p>
+	  If something goes wrong during the initialization
+          the function should return <c>{stop,Reason}</c>
+	  or <c>ignore</c>. See
+          <seealso marker="#start_link/3"><c>start_link/3,4</c></seealso>.
+	</p>
+	<p>
+	  This function may use
+	  <seealso marker="erts:erlang#throw/1"><c>throw/1</c></seealso>
+	  to return <c>Result</c>.
+	</p>
+      </desc>
+    </func>
+
+    <func>
+      <name>Module:StateName(EventType, EventContent, Data) ->
+      StateFunctionResult
+      </name>
+      <name>Module:handle_event(EventType, EventContent,
+      State, Data) -> HandleEventResult
+      </name>
+      <fsummary>Handle an event</fsummary>
+      <type>
+	<v>
+	  EventType =
+	  <seealso marker="#type-event_type">event_type()</seealso>
+	</v>
+	<v>EventContent = term()</v>
+	<v>
+	  State =
+	  <seealso marker="#type-state">state()</seealso>
+	</v>
+	<v>
+	  Data = NewData =
+	  <seealso marker="#type-data">data()</seealso>
+	</v>
+	<v>
+	  StateFunctionResult =
+	  <seealso marker="#type-state_function_result">
+	    state_function_result()
+	  </seealso>
+	</v>
+	<v>
+	  HandleEventResult =
+	  <seealso marker="#type-handle_event_result">
+	    handle_event_result()
+	  </seealso>
+	</v>
+      </type>
+      <desc>
+        <p>
+	  Whenever a <c>gen_statem</c> receives an event from
+          <seealso marker="#call/2"><c>call/2</c></seealso>,
+          <seealso marker="#cast/2"><c>cast/2</c></seealso> or
+	  as a normal process message one of these functions is called.  If
+	  <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
+	  is <c>state_functions</c> then <c>Module:StateName/3</c> is called,
+	  and if it is <c>handle_event_function</c>
+	  then <c>Module:handle_event/4</c> is called.
+	</p>
+	<p>
+	  If <c>EventType</c> is
+	  <seealso marker="#type-event_type"><c>{call,From}</c></seealso>
+	  the caller is waiting for a reply.  The reply can be sent
+	  from this or from any other
+	  <seealso marker="#state_function">state function</seealso>
+	  by returning with <c>{reply,From,Reply}</c> in
+	  <seealso marker="#type-action"><c>Actions</c></seealso>, in
+	  <seealso marker="#type-reply_action"><c>Replies</c></seealso>
+	  or by calling
+	  <seealso marker="#reply/2"><c>reply(From, Reply)</c></seealso>.
+	</p>
+	<p>
+	  If this function returns with a next state that
+	  does not match equal (<c>=/=</c>) to the current state
+	  all postponed events will be retried in the next state.
+	</p>
+	<p>
+	  The only difference between <c>StateFunctionResult</c> and
+	  <c>HandleEventResult</c> is that for <c>StateFunctionResult</c>
+	  the next state has to be an atom but for <c>HandleEventResult</c>
+	  there is no restriction on the next state.
+	</p>
+	<p>
+	  See <seealso marker="#type-action"><c>action()</c></seealso>
+	  for options that can be set and actions that can be done
+	  by <c>gen_statem</c> after returning from this function.
+	</p>
+	<p>
+	  These functions may use
+	  <seealso marker="erts:erlang#throw/1"><c>throw/1</c></seealso>,
+	  to return the result.
+	</p>
+      </desc>
+    </func>
+
+    <func>
+      <name>Module:terminate(Reason, State, Data) -> Ignored</name>
+      <fsummary>Clean up before termination</fsummary>
+      <type>
+        <v>Reason = normal | shutdown | {shutdown,term()} | term()</v>
+	<v>State = <seealso marker="#type-state">state()</seealso></v>
+	<v>Data = <seealso marker="#type-data">data()</seealso></v>
+	<v>Ignored = term()</v>
+      </type>
+      <desc>
+        <p>
+	  This function is called by a <c>gen_statem</c>
+	  when it is about to terminate. It should be the opposite of
+	  <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
+          and do any necessary cleaning up. When it returns,
+          the <c>gen_statem</c> terminates with <c>Reason</c>. The return
+          value is ignored.</p>
+        <p>
+	  <c>Reason</c> is a term denoting the stop reason and
+          <seealso marker="#type-state"><c>State</c></seealso>
+	  is the internal state of the <c>gen_statem</c>.
+	</p>
+        <p>
+	  <c>Reason</c> depends on why the <c>gen_statem</c>
+	  is terminating.
+          If it is because another callback function has returned a
+          stop tuple <c>{stop,Reason}</c> in
+	  <seealso marker="#type-action"><c>Actions</c></seealso>,
+	  <c>Reason</c> will have the value specified in that tuple.
+	  If it is due to a failure, <c>Reason</c> is the error reason.
+	</p>
+        <p>
+	  If the <c>gen_statem</c> is part of a supervision tree and is
+          ordered by its supervisor to terminate, this function will be
+          called with <c>Reason = shutdown</c> if the following
+          conditions apply:</p>
+        <list type="bulleted">
+          <item>
+	    the <c>gen_statem</c> has been set
+	    to trap exit signals, and
+	  </item>
+          <item>
+	    the shutdown strategy as defined in the supervisor's
+            child specification is an integer timeout value, not
+            <c>brutal_kill</c>.
+	  </item>
+        </list>
+        <p>
+	  Even if the <c>gen_statem</c> is <em>not</em>
+	  part of a supervision tree, this function will be called
+	  if it receives an <c>'EXIT'</c> message from its parent.
+	  <c>Reason</c> will be the same as
+	  in the <c>'EXIT'</c> message.
+	</p>
+        <p>
+	  Otherwise, the <c>gen_statem</c> will be immediately terminated.
+	</p>
+        <p>
+	  Note that for any other reason than <c>normal</c>,
+          <c>shutdown</c>, or <c>{shutdown,Term}</c>
+	  the <c>gen_statem</c> is assumed to terminate due to an error
+	  and an error report is issued using
+          <seealso marker="kernel:error_logger#format/2">
+	    <c>error_logger:format/2</c>.
+	  </seealso>
+	</p>
+	<p>
+	  This function may use
+	  <seealso marker="erts:erlang#throw/1"><c>throw/1</c></seealso>
+	  to return <c>Ignored</c>, which is ignored anyway.
+	</p>
+      </desc>
+    </func>
+
+    <func>
+      <name>Module:code_change(OldVsn, OldState, OldData, Extra) ->
+        Result
+      </name>
+      <fsummary>Update the internal state during upgrade/downgrade</fsummary>
+      <type>
+        <v>OldVsn = Vsn | {down,Vsn}</v>
+        <v>&nbsp;&nbsp;Vsn = term()</v>
+        <v>OldState = NewState = term()</v>
+        <v>Extra = term()</v>
+	<v>Result = {NewCallbackMode,NewState,NewData} | Reason</v>
+	<v>
+	  NewCallbackMode =
+	  <seealso marker="#type-callback_mode">callback_mode()</seealso>
+	</v>
+	<v>
+	  OldState = NewState =
+	  <seealso marker="#type-state">state()</seealso>
+	</v>
+	<v>
+	  OldData = NewData =
+	  <seealso marker="#type-data">data()</seealso>
+	</v>
+	<v>Reason = term()</v>
+      </type>
+      <desc>
+        <p>
+	  This function is called by a <c>gen_statem</c> when it should
+          update its internal state during a release upgrade/downgrade,
+          that is when the instruction <c>{update,Module,Change,...}</c>
+          where <c>Change={advanced,Extra}</c> is given in the
+	  <seealso marker="sasl:appup"><c>appup</c></seealso>
+	  file. See
+          <seealso marker="doc/design_principles:release_handling#instr">
+	    OTP Design Principles
+	  </seealso>
+          for more information.
+	</p>
+        <p>
+	  In the case of an upgrade, <c>OldVsn</c> is <c>Vsn</c>, and
+          in the case of a downgrade, <c>OldVsn</c> is
+          <c>{down,Vsn}</c>. <c>Vsn</c> is defined by the <c>vsn</c>
+          attribute(s) of the old version of the callback module
+          <c>Module</c>. If no such attribute is defined, the version
+          is the checksum of the BEAM file.
+	</p>
+	<note>
+	  <p>
+	    If you would dare to change
+	    <seealso marker="#type-callback_mode">
+	      <em>callback mode</em>
+	    </seealso>
+	    during release upgrade/downgrade, the upgrade is no problem
+	    since the new code surely knows what <em>callback mode</em>
+	    it needs, but for a downgrade this function will have to
+	    know from the <c>Extra</c> argument that comes from the
+	    <seealso marker="sasl:appup"><c>appup</c></seealso>
+	    file what <em>callback mode</em> the old code did use.
+	    It may also be possible to figure this out
+	    from the <c>{down,Vsn}</c> argument since <c>Vsn</c>
+	    in effect defines the old callback module version.
+	  </p>
+	</note>
+        <p>
+	  <c>OldState</c> and <c>OldData</c> is the internal state
+	  of the <c>gen_statem</c>.
+	</p>
+        <p>
+	  <c>Extra</c> is passed as-is from the <c>{advanced,Extra}</c>
+          part of the update instruction.
+	</p>
+        <p>
+	  If successful, the function shall return the updated
+          internal state in an
+	  <c>{NewCallbackMode,NewState,NewData}</c> tuple.
+	</p>
+	<p>
+	  If the function returns <c>Reason</c>, the ongoing
+	  upgrade will fail and roll back to the old release.</p>
+	<p>
+	  This function may use
+	  <seealso marker="erts:erlang#throw/1"><c>throw/1</c></seealso>
+	  to return <c>Result</c> or <c>Reason</c>.
+	</p>
+      </desc>
+    </func>
+
+    <func>
+      <name>Module:format_status(Opt, [PDict,State,Data]) ->
+        Status
+      </name>
+      <fsummary>Optional function for providing a term describing the
+        current <c>gen_statem</c> status</fsummary>
+      <type>
+        <v>Opt = normal | terminate</v>
+        <v>PDict = [{Key, Value}]</v>
+	<v>
+	  State =
+	  <seealso marker="#type-state">state()</seealso>
+	</v>
+	<v>
+	  Data =
+	  <seealso marker="#type-data">data()</seealso>
+	</v>
+	<v>Key = term()</v>
+	<v>Value = term()</v>
+        <v>Status = term()</v>
+      </type>
+      <desc>
+        <note>
+          <p>
+	    This callback is optional, so a callback module need not
+            export it. The <c>gen_statem</c> module provides a default
+            implementation of this function that returns
+            <c>{State,Data}</c>.  If this callback fails the default
+	    function will return <c>{State,Info}</c>
+	    where <c>Info</c> informs of the crash but no details,
+	    to hide possibly sensitive data.
+	  </p>
+        </note>
+        <p>This function is called by a <c>gen_statem</c> process when:</p>
+        <list type="bulleted">
+          <item>
+	    One of
+            <seealso marker="sys#get_status/1">
+	      <c>sys:get_status/1,2</c>
+	    </seealso>
+            is invoked to get the <c>gen_statem</c> status. <c>Opt</c> is set
+            to the atom <c>normal</c> for this case.
+	  </item>
+          <item>
+	    The <c>gen_statem</c> terminates abnormally and logs an error.
+	    <c>Opt</c> is set to the atom <c>terminate</c> for this case.
+	  </item>
+        </list>
+        <p>
+	  This function is useful for customising the form and
+          appearance of the <c>gen_statem</c> status for these cases. A
+          callback module wishing to customise the
+          <seealso marker="sys#get_status/1">
+	    <c>sys:get_status/1,2</c>
+	  </seealso>
+	  return value as well as how
+          its status appears in termination error logs exports an
+          instance of <c>format_status/2</c> that returns a term
+          describing the current status of the <c>gen_statem</c>.
+	</p>
+        <p>
+	  <c>PDict</c> is the current value of the <c>gen_statem</c>'s
+          process dictionary.
+	</p>
+        <p>
+	  <seealso marker="#type-state"><c>State</c></seealso>
+	  is the internal state of the <c>gen_statem</c>.
+	</p>
+        <p>
+	  <seealso marker="#type-data"><c>Data</c></seealso>
+	  is the internal server data of the <c>gen_statem</c>.
+	</p>
+        <p>
+	  The function should return <c>Status</c>, a term that
+          customises the details of the current state and status of
+          the <c>gen_statem</c>. There are no restrictions on the
+          form <c>Status</c> can take, but for the
+          <seealso marker="sys#get_status/1">
+	    <c>sys:get_status/1,2</c>
+	  </seealso>
+	  case (when <c>Opt</c>
+          is <c>normal</c>), the recommended form for
+          the <c>Status</c> value is <c>[{data, [{"State",
+          Term}]}]</c> where <c>Term</c> provides relevant details of
+          the <c>gen_statem</c> state. Following this recommendation isn't
+          required, but doing so will make the callback module status
+          consistent with the rest of the
+          <seealso marker="sys#get_status/1">
+	    <c>sys:get_status/1,2</c>
+	  </seealso>
+	  return value.
+	</p>
+        <p>
+	  One use for this function is to return compact alternative
+          state representations to avoid having large state terms
+          printed in logfiles.  Another is to hide sensitive data from
+	  being written to the error log.
+	</p>
+	<p>
+	  This function may use
+	  <seealso marker="erts:erlang#throw/1"><c>throw/1</c></seealso>
+	  to return <c>Status</c>.
+	</p>
+      </desc>
+    </func>
+
+  </funcs>
+
+  <section>
+    <title>SEE ALSO</title>
+    <p><seealso marker="gen_event"><c>gen_event(3)</c></seealso>,
+      <seealso marker="gen_fsm"><c>gen_fsm(3)</c></seealso>,
+      <seealso marker="gen_server"><c>gen_server(3)</c></seealso>,
+      <seealso marker="supervisor"><c>supervisor(3)</c></seealso>,
+      <seealso marker="proc_lib"><c>proc_lib(3)</c></seealso>,
+      <seealso marker="sys"><c>sys(3)</c></seealso></p>
+  </section>
+</erlref>
diff --git a/lib/stdlib/doc/src/proc_lib.xml b/lib/stdlib/doc/src/proc_lib.xml
index bef037076b..245580b1ba 100644
--- a/lib/stdlib/doc/src/proc_lib.xml
+++ b/lib/stdlib/doc/src/proc_lib.xml
@@ -34,9 +34,9 @@
     <p>This module is used to start processes adhering to
       the <seealso marker="doc/design_principles:des_princ">OTP Design Principles</seealso>. Specifically, the functions in this
       module are used by the OTP standard behaviors (<c>gen_server</c>,
-      <c>gen_fsm</c>, ...) when starting new processes. The functions
-      can also be used to start <em>special processes</em>, user
-      defined processes which comply to the OTP design principles. See
+      <c>gen_fsm</c>, <c>gen_statem</c>, ...) when starting new processes.
+      The functions can also be used to start <em>special processes</em>,
+      user defined processes which comply to the OTP design principles. See
       <seealso marker="doc/design_principles:spec_proc">Sys and Proc_Lib</seealso> in OTP Design Principles for an example.</p>
     <p>Some useful information is initialized when a process starts.
       The registered names, or the process identifiers, of the parent
diff --git a/lib/stdlib/doc/src/ref_man.xml b/lib/stdlib/doc/src/ref_man.xml
index 82ad78e675..404873ea32 100644
--- a/lib/stdlib/doc/src/ref_man.xml
+++ b/lib/stdlib/doc/src/ref_man.xml
@@ -4,7 +4,7 @@
 <application xmlns:xi="http://www.w3.org/2001/XInclude">
   <header>
     <copyright>
-      <year>1996</year><year>2015</year>
+      <year>1996</year><year>2016</year>
       <holder>Ericsson AB. All Rights Reserved.</holder>
     </copyright>
     <legalnotice>
@@ -66,6 +66,7 @@
   <xi:include href="gen_event.xml"/>
   <xi:include href="gen_fsm.xml"/>
   <xi:include href="gen_server.xml"/>
+  <xi:include href="gen_statem.xml"/>
   <xi:include href="io.xml"/>
   <xi:include href="io_lib.xml"/>
   <xi:include href="lib.xml"/>
diff --git a/lib/stdlib/doc/src/specs.xml b/lib/stdlib/doc/src/specs.xml
index 0418bf7b22..45b207b13d 100644
--- a/lib/stdlib/doc/src/specs.xml
+++ b/lib/stdlib/doc/src/specs.xml
@@ -30,6 +30,7 @@
   <xi:include href="../specs/specs_gen_event.xml"/>
   <xi:include href="../specs/specs_gen_fsm.xml"/>
   <xi:include href="../specs/specs_gen_server.xml"/>
+  <xi:include href="../specs/specs_gen_statem.xml"/>
   <xi:include href="../specs/specs_io.xml"/>
   <xi:include href="../specs/specs_io_lib.xml"/>
   <xi:include href="../specs/specs_lib.xml"/>
diff --git a/lib/stdlib/doc/src/supervisor.xml b/lib/stdlib/doc/src/supervisor.xml
index 421306283c..29e5a732d5 100644
--- a/lib/stdlib/doc/src/supervisor.xml
+++ b/lib/stdlib/doc/src/supervisor.xml
@@ -34,8 +34,8 @@
     <p>A behaviour module for implementing a supervisor, a process which
       supervises other processes called child processes. A child
       process can either be another supervisor or a worker process.
-      Worker processes are normally implemented using one of
-      the <c>gen_event</c>, <c>gen_fsm</c>, or <c>gen_server</c>
+      Worker processes are normally implemented using one of the
+      <c>gen_event</c>, <c>gen_fsm</c>, <c>gen_statem</c> or <c>gen_server</c>
       behaviours. A supervisor implemented using this module will have
       a standard set of interface functions and include functionality
       for tracing and error reporting. Supervisors are used to build a
@@ -221,7 +221,8 @@
         <p><c>modules</c> is used by the release handler during code
           replacement to determine which processes are using a certain
           module. As a rule of thumb, if the child process is a
-          <c>supervisor</c>, <c>gen_server</c>, or <c>gen_fsm</c>,
+          <c>supervisor</c>, <c>gen_server</c>,
+	  <c>gen_fsm</c> or <c>gen_statem</c>
 	  this should be a list with one element <c>[Module]</c>,
 	  where <c>Module</c> is the callback module. If the child
 	  process is an event manager (<c>gen_event</c>) with a
@@ -636,6 +637,7 @@
     <title>SEE ALSO</title>
     <p><seealso marker="gen_event">gen_event(3)</seealso>, 
       <seealso marker="gen_fsm">gen_fsm(3)</seealso>, 
+      <seealso marker="gen_statem">gen_statem(3)</seealso>,
       <seealso marker="gen_server">gen_server(3)</seealso>, 
       <seealso marker="sys">sys(3)</seealso></p>
   </section>
diff --git a/lib/stdlib/doc/src/sys.xml b/lib/stdlib/doc/src/sys.xml
index d80ec4dfa6..2255395f46 100644
--- a/lib/stdlib/doc/src/sys.xml
+++ b/lib/stdlib/doc/src/sys.xml
@@ -217,14 +217,18 @@
           processes. For example, a <c>gen_server</c> process returns
           the callback module's state, a <c>gen_fsm</c> process
           returns information such as its current state name and state data,
-          and a <c>gen_event</c> process returns information about each of its
+	  a <c>gen_statem</c> process returns information about
+	  its current state and data, and a <c>gen_event</c> process
+	  returns information about each of its
           registered handlers. Callback modules for <c>gen_server</c>,
-          <c>gen_fsm</c>, and <c>gen_event</c> can also customise the value
+          <c>gen_fsm</c>, <c>gen_statem</c> and <c>gen_event</c>
+	  can also customise the value
           of <c><anno>Misc</anno></c> by exporting a <c>format_status/2</c>
           function that contributes module-specific information;
-          see <seealso marker="gen_server#Module:format_status/2">gen_server:format_status/2</seealso>,
-          <seealso marker="gen_fsm#Module:format_status/2">gen_fsm:format_status/2</seealso>, and
-          <seealso marker="gen_event#Module:format_status/2">gen_event:format_status/2</seealso>
+          see <seealso marker="gen_server#Module:format_status/2">gen_server format_status/2</seealso>,
+          <seealso marker="gen_fsm#Module:format_status/2">gen_fsm format_status/2</seealso>,
+          <seealso marker="gen_statem#Module:format_status/2">gen_statem format_status/2</seealso>, and
+          <seealso marker="gen_event#Module:format_status/2">gen_event format_status/2</seealso>
           for more details.</p>
       </desc>
     </func>
@@ -245,6 +249,8 @@
           processes. For a <c>gen_server</c> process, the returned <c><anno>State</anno></c>
           is simply the callback module's state. For a <c>gen_fsm</c> process,
           <c><anno>State</anno></c> is the tuple <c>{CurrentStateName, CurrentStateData}</c>.
+	  For a <c>gen_statem</c> process <c><anno>State</anno></c> is
+	  the tuple <c>{CurrentState,CurrentData}.</c>
           For a <c>gen_event</c> process, <c><anno>State</anno></c> a list of tuples,
           where each tuple corresponds to an event handler registered in the process and contains
           <c>{Module, Id, HandlerState}</c>, where <c>Module</c> is the event handler's module name,
@@ -263,8 +269,9 @@
           details of the exception.</p>
         <p>The <c>system_get_state/1</c> function is primarily useful for user-defined
           behaviours and modules that implement OTP <seealso marker="#special_process">special
-          processes</seealso>. The <c>gen_server</c>, <c>gen_fsm</c>, and <c>gen_event</c> OTP
-          behaviour modules export this function, and so callback modules for those behaviours
+          processes</seealso>. The <c>gen_server</c>, <c>gen_fsm</c>,
+	  <c>gen_statem</c> and <c>gen_event</c> OTP
+          behaviour modules export this function, so callback modules for those behaviours
           need not supply their own.</p>
         <p>To obtain more information about a process, including its state, see
           <seealso marker="#get_status-1">get_status/1</seealso> and
@@ -290,6 +297,8 @@
           <c>gen_fsm</c> process, <c><anno>State</anno></c> is the tuple
           <c>{CurrentStateName, CurrentStateData}</c>, and <c><anno>NewState</anno></c>
           is a similar tuple that may contain a new state name, new state data, or both.
+	  The same applies for a <c>gen_statem</c> process but
+	  it names the tuple fields <c>{CurrentState,CurrentData}</c>.
           For a <c>gen_event</c> process, <c><anno>State</anno></c> is the tuple
           <c>{Module, Id, HandlerState}</c> where <c>Module</c> is the event handler's module name,
           <c>Id</c> is the handler's ID (which is the value <c>false</c> if it was registered without
@@ -304,7 +313,8 @@
           state, then regardless of process type, it may simply return its <c><anno>State</anno></c>
           argument.</p>
         <p>If a <c><anno>StateFun</anno></c> function crashes or throws an exception, then
-          for <c>gen_server</c> and <c>gen_fsm</c> processes, the original state of the process is
+          for <c>gen_server</c>, <c>gen_fsm</c> or <c>gen_statem</c> processes,
+	  the original state of the process is
           unchanged. For <c>gen_event</c> processes, a crashing or failing <c><anno>StateFun</anno></c>
           function means that only the state of the particular event handler it was working on when it
           failed or crashed is unchanged; it can still succeed in changing the states of other event
@@ -329,7 +339,8 @@
           <c>{callback_failed, StateFun, {Class, Reason}}</c>.</p>
         <p>The <c>system_replace_state/2</c> function is primarily useful for user-defined behaviours and
           modules that implement OTP <seealso marker="#special_process">special processes</seealso>. The
-          <c>gen_server</c>, <c>gen_fsm</c>, and <c>gen_event</c> OTP behaviour modules export this function,
+          <c>gen_server</c>, <c>gen_fsm</c>, <c>gen_statem</c> and
+	  <c>gen_event</c> OTP behaviour modules export this function,
           and so callback modules for those behaviours need not supply their own.</p>
       </desc>
     </func>
diff --git a/lib/stdlib/src/Makefile b/lib/stdlib/src/Makefile
index 9f4a446ea0..302834f9d0 100644
--- a/lib/stdlib/src/Makefile
+++ b/lib/stdlib/src/Makefile
@@ -1,7 +1,7 @@
 #
 # %CopyrightBegin%
 #
-# Copyright Ericsson AB 1996-2015. All Rights Reserved.
+# Copyright Ericsson AB 1996-2016. All Rights Reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -85,6 +85,7 @@ MODULES= \
 	gen_event \
 	gen_fsm \
 	gen_server \
+	gen_statem \
 	io \
 	io_lib \
 	io_lib_format \
diff --git a/lib/stdlib/src/gen.erl b/lib/stdlib/src/gen.erl
index 9811a211ae..597830cf9a 100644
--- a/lib/stdlib/src/gen.erl
+++ b/lib/stdlib/src/gen.erl
@@ -26,7 +26,8 @@
 %%%
 %%% The standard behaviour should export init_it/6.
 %%%-----------------------------------------------------------------
--export([start/5, start/6, debug_options/1,
+-export([start/5, start/6, debug_options/2,
+	 name/1, unregister_name/1, get_proc_name/1, get_parent/0,
 	 call/3, call/4, reply/2, stop/1, stop/3]).
 
 -export([init_it/6, init_it/7]).
@@ -124,7 +125,7 @@ init_it(GenMod, Starter, Parent, Mod, Args, Options) ->
     init_it2(GenMod, Starter, Parent, self(), Mod, Args, Options).
 
 init_it(GenMod, Starter, Parent, Name, Mod, Args, Options) ->
-    case name_register(Name) of
+    case register_name(Name) of
 	true ->
 	    init_it2(GenMod, Starter, Parent, Name, Mod, Args, Options);
 	{false, Pid} ->
@@ -297,19 +298,19 @@ where({global, Name}) -> global:whereis_name(Name);
 where({via, Module, Name}) -> Module:whereis_name(Name);
 where({local, Name})  -> whereis(Name).
 
-name_register({local, Name} = LN) ->
+register_name({local, Name} = LN) ->
     try register(Name, self()) of
 	true -> true
     catch
 	error:_ ->
 	    {false, where(LN)}
     end;
-name_register({global, Name} = GN) ->
+register_name({global, Name} = GN) ->
     case global:register_name(Name, self()) of
 	yes -> true;
 	no -> {false, where(GN)}
     end;
-name_register({via, Module, Name} = GN) ->
+register_name({via, Module, Name} = GN) ->
     case Module:register_name(Name, self()) of
 	yes ->
 	    true;
@@ -317,34 +318,108 @@ name_register({via, Module, Name} = GN) ->
 	    {false, where(GN)}
     end.
 
+name({local,Name}) -> Name;
+name({global,Name}) -> Name;
+name({via,_, Name}) -> Name;
+name(Pid) when is_pid(Pid) -> Pid.
+
+unregister_name({local,Name}) ->
+    try unregister(Name) of
+	_ -> ok
+    catch
+	_:_ -> ok
+    end;
+unregister_name({global,Name}) ->
+    _ = global:unregister_name(Name),
+    ok;
+unregister_name({via, Mod, Name}) ->
+    _ = Mod:unregister_name(Name),
+    ok;
+unregister_name(Pid) when is_pid(Pid) ->
+    ok.
+
+get_proc_name(Pid) when is_pid(Pid) ->
+    Pid;
+get_proc_name({local, Name}) ->
+    case process_info(self(), registered_name) of
+	{registered_name, Name} ->
+	    Name;
+	{registered_name, _Name} ->
+	    exit(process_not_registered);
+	[] ->
+	    exit(process_not_registered)
+    end;
+get_proc_name({global, Name}) ->
+    case global:whereis_name(Name) of
+	undefined ->
+	    exit(process_not_registered_globally);
+	Pid when Pid =:= self() ->
+	    Name;
+	_Pid ->
+	    exit(process_not_registered_globally)
+    end;
+get_proc_name({via, Mod, Name}) ->
+    case Mod:whereis_name(Name) of
+	undefined ->
+	    exit({process_not_registered_via, Mod});
+	Pid when Pid =:= self() ->
+	    Name;
+	_Pid ->
+	    exit({process_not_registered_via, Mod})
+    end.
+
+get_parent() ->
+    case get('$ancestors') of
+	[Parent | _] when is_pid(Parent) ->
+	    Parent;
+	[Parent | _] when is_atom(Parent) ->
+	    name_to_pid(Parent);
+	_ ->
+	    exit(process_was_not_started_by_proc_lib)
+    end.
+
+name_to_pid(Name) ->
+    case whereis(Name) of
+	undefined ->
+	    case global:whereis_name(Name) of
+		undefined ->
+		    exit(could_not_find_registered_name);
+		Pid ->
+		    Pid
+	    end;
+	Pid ->
+	    Pid
+    end.
+
 
 timeout(Options) ->
-    case opt(timeout, Options) of
-	{ok, Time} ->
+    case lists:keyfind(timeout, 1, Options) of
+	{_,Time} ->
 	    Time;
-	_ ->
+	false ->
 	    infinity
     end.
 
 spawn_opts(Options) ->
-    case opt(spawn_opt, Options) of
-	{ok, Opts} ->
+    case lists:keyfind(spawn_opt, 1, Options) of
+	{_,Opts} ->
 	    Opts;
-	_ ->
+	false ->
 	    []
     end.
 
-opt(Op, [{Op, Value}|_]) ->
-    {ok, Value};
-opt(Op, [_|Options]) ->
-    opt(Op, Options);
-opt(_, []) ->
-    false.
-
-debug_options(Opts) ->
-    case opt(debug, Opts) of
-	{ok, Options} -> sys:debug_options(Options);
-	_ -> []
+debug_options(Name, Opts) ->
+    case lists:keyfind(debug, 1, Opts) of
+	{_,Options} ->
+	    try sys:debug_options(Options)
+	    catch _:_ ->
+		    error_logger:format(
+		      "~p: ignoring erroneous debug options - ~p~n",
+		      [Name,Options]),
+		    []
+	    end;
+	false ->
+	    []
     end.
 
 format_status_header(TagLine, Pid) when is_pid(Pid) ->
diff --git a/lib/stdlib/src/gen_event.erl b/lib/stdlib/src/gen_event.erl
index 1303036168..ccacf658e9 100644
--- a/lib/stdlib/src/gen_event.erl
+++ b/lib/stdlib/src/gen_event.erl
@@ -147,16 +147,11 @@ init_it(Starter, self, Name, Mod, Args, Options) ->
     init_it(Starter, self(), Name, Mod, Args, Options);
 init_it(Starter, Parent, Name0, _, _, Options) ->
     process_flag(trap_exit, true),
-    Debug = gen:debug_options(Options),
+    Name = gen:name(Name0),
+    Debug = gen:debug_options(Name, Options),
     proc_lib:init_ack(Starter, {ok, self()}),
-    Name = name(Name0),
     loop(Parent, Name, [], Debug, false).
 
-name({local,Name}) -> Name;
-name({global,Name}) -> Name;
-name({via,_, Name}) -> Name;
-name(Pid) when is_pid(Pid) -> Pid.
-
 -spec add_handler(emgr_ref(), handler(), term()) -> term().
 add_handler(M, Handler, Args) -> rpc(M, {add_handler, Handler, Args}).
 
diff --git a/lib/stdlib/src/gen_fsm.erl b/lib/stdlib/src/gen_fsm.erl
index 2b4b76105e..6e7528fd98 100644
--- a/lib/stdlib/src/gen_fsm.erl
+++ b/lib/stdlib/src/gen_fsm.erl
@@ -305,64 +305,11 @@ enter_loop(Mod, Options, StateName, StateData, Timeout) ->
     enter_loop(Mod, Options, StateName, StateData, self(), Timeout).
 
 enter_loop(Mod, Options, StateName, StateData, ServerName, Timeout) ->
-    Name = get_proc_name(ServerName),
-    Parent = get_parent(),
-    Debug = gen:debug_options(Options),
+    Name = gen:get_proc_name(ServerName),
+    Parent = gen:get_parent(),
+    Debug = gen:debug_options(Name, Options),
     loop(Parent, Name, StateName, StateData, Mod, Timeout, Debug).
 
-get_proc_name(Pid) when is_pid(Pid) ->
-    Pid;
-get_proc_name({local, Name}) ->
-    case process_info(self(), registered_name) of
-	{registered_name, Name} ->
-	    Name;
-	{registered_name, _Name} ->
-	    exit(process_not_registered);
-	[] ->
-	    exit(process_not_registered)
-    end;
-get_proc_name({global, Name}) ->
-    case global:whereis_name(Name) of
-	undefined ->
-	    exit(process_not_registered_globally);
-	Pid when Pid =:= self() ->
-	    Name;
-	_Pid ->
-	    exit(process_not_registered_globally)
-    end;
-get_proc_name({via, Mod, Name}) ->
-    case Mod:whereis_name(Name) of
-	undefined ->
-	    exit({process_not_registered_via, Mod});
-	Pid when Pid =:= self() ->
-	    Name;
-	_Pid ->
-	    exit({process_not_registered_via, Mod})
-    end.
-
-get_parent() ->
-    case get('$ancestors') of
-	[Parent | _] when is_pid(Parent) ->
-	    Parent;
-	[Parent | _] when is_atom(Parent) ->
-	    name_to_pid(Parent);
-	_ ->
-	    exit(process_was_not_started_by_proc_lib)
-    end.
-
-name_to_pid(Name) ->
-    case whereis(Name) of
-	undefined ->
-	    case global:whereis_name(Name) of
-		undefined ->
-		    exit(could_not_find_registered_name);
-		Pid ->
-		    Pid
-	    end;
-	Pid ->
-	    Pid
-    end.
-
 %%% ---------------------------------------------------
 %%% Initiate the new process.
 %%% Register the name using the Rfunc function
@@ -373,8 +320,8 @@ name_to_pid(Name) ->
 init_it(Starter, self, Name, Mod, Args, Options) ->
     init_it(Starter, self(), Name, Mod, Args, Options);
 init_it(Starter, Parent, Name0, Mod, Args, Options) ->
-    Name = name(Name0),
-    Debug = gen:debug_options(Options),
+    Name = gen:name(Name0),
+    Debug = gen:debug_options(Name, Options),
     case catch Mod:init(Args) of
 	{ok, StateName, StateData} ->
 	    proc_lib:init_ack(Starter, {ok, self()}), 	    
@@ -383,15 +330,15 @@ init_it(Starter, Parent, Name0, Mod, Args, Options) ->
 	    proc_lib:init_ack(Starter, {ok, self()}), 	    
 	    loop(Parent, Name, StateName, StateData, Mod, Timeout, Debug);
 	{stop, Reason} ->
-	    unregister_name(Name0),
+	    gen:unregister_name(Name0),
 	    proc_lib:init_ack(Starter, {error, Reason}),
 	    exit(Reason);
 	ignore ->
-	    unregister_name(Name0),
+	    gen:unregister_name(Name0),
 	    proc_lib:init_ack(Starter, ignore),
 	    exit(normal);
 	{'EXIT', Reason} ->
-	    unregister_name(Name0),
+	    gen:unregister_name(Name0),
 	    proc_lib:init_ack(Starter, {error, Reason}),
 	    exit(Reason);
 	Else ->
@@ -400,20 +347,6 @@ init_it(Starter, Parent, Name0, Mod, Args, Options) ->
 	    exit(Error)
     end.
 
-name({local,Name}) -> Name;
-name({global,Name}) -> Name;
-name({via,_, Name}) -> Name;
-name(Pid) when is_pid(Pid) -> Pid.
-
-unregister_name({local,Name}) ->
-    _ = (catch unregister(Name));
-unregister_name({global,Name}) ->
-    _ = global:unregister_name(Name);
-unregister_name({via, Mod, Name}) ->
-    _ = Mod:unregister_name(Name);
-unregister_name(Pid) when is_pid(Pid) ->
-    Pid.
-
 %%-----------------------------------------------------------------
 %% The MAIN loop
 %%-----------------------------------------------------------------
diff --git a/lib/stdlib/src/gen_server.erl b/lib/stdlib/src/gen_server.erl
index fa6fff6dca..5800aca66f 100644
--- a/lib/stdlib/src/gen_server.erl
+++ b/lib/stdlib/src/gen_server.erl
@@ -304,9 +304,9 @@ enter_loop(Mod, Options, State, Timeout) ->
     enter_loop(Mod, Options, State, self(), Timeout).
 
 enter_loop(Mod, Options, State, ServerName, Timeout) ->
-    Name = get_proc_name(ServerName),
-    Parent = get_parent(),
-    Debug = debug_options(Name, Options),
+    Name = gen:get_proc_name(ServerName),
+    Parent = gen:get_parent(),
+    Debug = gen:debug_options(Name, Options),
     loop(Parent, Name, State, Mod, Timeout, Debug).
 
 %%%========================================================================
@@ -323,8 +323,8 @@ enter_loop(Mod, Options, State, ServerName, Timeout) ->
 init_it(Starter, self, Name, Mod, Args, Options) ->
     init_it(Starter, self(), Name, Mod, Args, Options);
 init_it(Starter, Parent, Name0, Mod, Args, Options) ->
-    Name = name(Name0),
-    Debug = debug_options(Name, Options),
+    Name = gen:name(Name0),
+    Debug = gen:debug_options(Name, Options),
     case catch Mod:init(Args) of
 	{ok, State} ->
 	    proc_lib:init_ack(Starter, {ok, self()}), 	    
@@ -339,15 +339,15 @@ init_it(Starter, Parent, Name0, Mod, Args, Options) ->
 	    %% (Otherwise, the parent process could get
 	    %% an 'already_started' error if it immediately
 	    %% tried starting the process again.)
-	    unregister_name(Name0),
+	    gen:unregister_name(Name0),
 	    proc_lib:init_ack(Starter, {error, Reason}),
 	    exit(Reason);
 	ignore ->
-	    unregister_name(Name0),
+	    gen:unregister_name(Name0),
 	    proc_lib:init_ack(Starter, ignore),
 	    exit(normal);
 	{'EXIT', Reason} ->
-	    unregister_name(Name0),
+	    gen:unregister_name(Name0),
 	    proc_lib:init_ack(Starter, {error, Reason}),
 	    exit(Reason);
 	Else ->
@@ -356,20 +356,6 @@ init_it(Starter, Parent, Name0, Mod, Args, Options) ->
 	    exit(Error)
     end.
 
-name({local,Name}) -> Name;
-name({global,Name}) -> Name;
-name({via,_, Name}) -> Name;
-name(Pid) when is_pid(Pid) -> Pid.
-
-unregister_name({local,Name}) ->
-    _ = (catch unregister(Name));
-unregister_name({global,Name}) ->
-    _ = global:unregister_name(Name);
-unregister_name({via, Mod, Name}) ->
-    _ = Mod:unregister_name(Name);
-unregister_name(Pid) when is_pid(Pid) ->
-    Pid.
-    
 %%%========================================================================
 %%% Internal functions
 %%%========================================================================
@@ -858,86 +844,6 @@ error_info(Reason, Name, Msg, State, Debug) ->
     sys:print_log(Debug),
     ok.
 
-%%% ---------------------------------------------------
-%%% Misc. functions.
-%%% ---------------------------------------------------
-
-opt(Op, [{Op, Value}|_]) ->
-    {ok, Value};
-opt(Op, [_|Options]) ->
-    opt(Op, Options);
-opt(_, []) ->
-    false.
-
-debug_options(Name, Opts) ->
-    case opt(debug, Opts) of
-	{ok, Options} -> dbg_opts(Name, Options);
-	_ -> []
-    end.
-
-dbg_opts(Name, Opts) ->
-    case catch sys:debug_options(Opts) of
-	{'EXIT',_} ->
-	    format("~p: ignoring erroneous debug options - ~p~n",
-		   [Name, Opts]),
-	    [];
-	Dbg ->
-	    Dbg
-    end.
-
-get_proc_name(Pid) when is_pid(Pid) ->
-    Pid;
-get_proc_name({local, Name}) ->
-    case process_info(self(), registered_name) of
-	{registered_name, Name} ->
-	    Name;
-	{registered_name, _Name} ->
-	    exit(process_not_registered);
-	[] ->
-	    exit(process_not_registered)
-    end;    
-get_proc_name({global, Name}) ->
-    case global:whereis_name(Name) of
-	undefined ->
-	    exit(process_not_registered_globally);
-	Pid when Pid =:= self() ->
-	    Name;
-	_Pid ->
-	    exit(process_not_registered_globally)
-    end;
-get_proc_name({via, Mod, Name}) ->
-    case Mod:whereis_name(Name) of
-	undefined ->
-	    exit({process_not_registered_via, Mod});
-	Pid when Pid =:= self() ->
-	    Name;
-	_Pid ->
-	    exit({process_not_registered_via, Mod})
-    end.
-
-get_parent() ->
-    case get('$ancestors') of
-	[Parent | _] when is_pid(Parent)->
-            Parent;
-        [Parent | _] when is_atom(Parent)->
-            name_to_pid(Parent);
-	_ ->
-	    exit(process_was_not_started_by_proc_lib)
-    end.
-
-name_to_pid(Name) ->
-    case whereis(Name) of
-	undefined ->
-	    case global:whereis_name(Name) of
-		undefined ->
-		    exit(could_not_find_registered_name);
-		Pid ->
-		    Pid
-	    end;
-	Pid ->
-	    Pid
-    end.
-
 %%-----------------------------------------------------------------
 %% Status information
 %%-----------------------------------------------------------------
diff --git a/lib/stdlib/src/gen_statem.erl b/lib/stdlib/src/gen_statem.erl
new file mode 100644
index 0000000000..c85c521d8e
--- /dev/null
+++ b/lib/stdlib/src/gen_statem.erl
@@ -0,0 +1,1257 @@
+%%
+%% %CopyrightBegin%
+%%
+%% Copyright Ericsson AB 2016. All Rights Reserved.
+%%
+%% Licensed under the Apache License, Version 2.0 (the "License");
+%% you may not use this file except in compliance with the License.
+%% You may obtain a copy of the License at
+%%
+%%     http://www.apache.org/licenses/LICENSE-2.0
+%%
+%% Unless required by applicable law or agreed to in writing, software
+%% distributed under the License is distributed on an "AS IS" BASIS,
+%% WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+%% See the License for the specific language governing permissions and
+%% limitations under the License.
+%%
+%% %CopyrightEnd%
+%%
+-module(gen_statem).
+
+%% API
+-export(
+   [start/3,start/4,start_link/3,start_link/4,
+    stop/1,stop/3,
+    cast/2,call/2,call/3,
+    enter_loop/5,enter_loop/6,enter_loop/7,
+    reply/1,reply/2]).
+
+%% gen callbacks
+-export(
+   [init_it/6]).
+
+%% sys callbacks
+-export(
+   [system_continue/3,
+    system_terminate/4,
+    system_code_change/4,
+    system_get_state/1,
+    system_replace_state/2,
+    format_status/2]).
+
+%% Internal callbacks
+-export(
+   [wakeup_from_hibernate/3]).
+
+%% Type exports for templates
+-export_type(
+   [event_type/0,
+    callback_mode/0,
+    state_function_result/0,
+    handle_event_result/0,
+    action/0]).
+
+%% Fix problem for doc build
+-export_type([transition_option/0]).
+
+%%%==========================================================================
+%%% Interface functions.
+%%%==========================================================================
+
+-type from() ::
+	{To :: pid(), Tag :: term()}. % Reply-to specifier for call
+
+-type state() ::
+	state_name() | % For state callback function StateName/5
+	term(). % For state callback function handle_event/5
+
+-type state_name() :: atom().
+
+-type data() :: term().
+
+-type event_type() ::
+	{'call',From :: from()} | 'cast' |
+	'info' | 'timeout' | 'internal'.
+
+-type callback_mode() :: 'state_functions' | 'handle_event_function'.
+
+-type transition_option() ::
+	postpone() | hibernate() | event_timeout().
+-type postpone() ::
+	%% If 'true' postpone the current event
+	%% and retry it when the state changes (=/=)
+	boolean().
+-type hibernate() ::
+	%% If 'true' hibernate the server instead of going into receive
+	boolean().
+-type event_timeout() ::
+	%% Generate a ('timeout', EventContent, ...) event after Time
+	%% unless some other event is delivered
+	Time :: timeout().
+
+-type action() ::
+	%% During a state change:
+	%% * NextState and NewData are set.
+	%% * All action()s are executed in order of apperance.
+	%% * Postponing the current event is performed
+	%%   iff 'postpone' is 'true'.
+	%% * A state timer is started iff 'timeout' is set.
+	%% * Pending events are processed or if there are
+	%%   no pending events the server goes into receive
+	%%   or hibernate (iff 'hibernate' is 'true')
+	%%
+	%% These action()s are executed in order of appearence
+	%% in the containing list. The ones that set options
+	%% will override any previous so the last of each kind wins.
+	%%
+	'postpone' |  % Set the postpone option
+	{'postpone', Postpone :: postpone()} |
+	%%
+	'hibernate' | % Set the hibernate option
+	{'hibernate', Hibernate :: hibernate()} |
+	%%
+	(Timeout :: event_timeout()) | % {timeout,Timeout}
+	{'timeout', % Set the event timeout option
+	 Time :: event_timeout(), EventContent :: term()} |
+	%%
+	reply_action() |
+	%%
+	%% All 'next_event' events are kept in a list and then
+	%% inserted at state changes so the first in the
+	%% action() list is the first to be delivered.
+	{'next_event', % Insert event as the next to handle
+	 EventType :: event_type(),
+	 EventContent :: term()}.
+-type reply_action() ::
+	{'reply', % Reply to a caller
+	 From :: from(), Reply :: term()}.
+
+-type state_function_result() ::
+	{'next_state', % {next_state,NextStateName,NewData,[]}
+	 NextStateName :: state_name(),
+	 NewData :: data()} |
+	{'next_state', % State transition, maybe to the same state
+	 NextStateName :: state_name(),
+	 NewData :: data(),
+	 Actions :: [action()] | action()} |
+	common_state_callback_result().
+-type handle_event_result() ::
+	{'next_state', % {next_state,NextState,NewData,[]}
+	 NextState :: state(),
+	 NewData :: data()} |
+	{'next_state', % State transition, maybe to the same state
+	 NextState :: state(),
+	 NewData :: data(),
+	 Actions :: [action()] | action()} |
+	common_state_callback_result().
+-type common_state_callback_result() ::
+	'stop' | % {stop,normal}
+	{'stop', % Stop the server
+	 Reason :: term()} |
+	{'stop', % Stop the server
+	 Reason :: term(),
+	 NewData :: data()} |
+	{'stop_and_reply', % Reply then stop the server
+	 Reason :: term(),
+	 Replies :: [reply_action()] | reply_action()} |
+	{'stop_and_reply', % Reply then stop the server
+	 Reason :: term(),
+	 Replies :: [reply_action()] | reply_action(),
+	 NewData :: data()} |
+	{'keep_state', % {keep_state,NewData,[]}
+	 NewData :: data()} |
+	{'keep_state', % Keep state, change data
+	 NewData :: data(),
+	 Actions :: [action()] | action()} |
+	'keep_state_and_data' | % {keep_state_and_data,[]}
+	{'keep_state_and_data', % Keep state and data -> only actions
+	 Actions :: [action()] | action()}.
+
+
+%% The state machine init function.  It is called only once and
+%% the server is not running until this function has returned
+%% an {ok, ...} tuple.  Thereafter the state callbacks are called
+%% for all events to this server.
+-callback init(Args :: term()) ->
+    {callback_mode(), state(), data()} |
+    {callback_mode(), state(), data(), [action()] | action()} |
+    'ignore' |
+    {'stop', Reason :: term()}.
+
+%% Example state callback for callback_mode() =:= state_functions
+%% state name 'state_name'.
+%%
+%% In this mode all states has to be type state_name() i.e atom().
+%%
+%% Note that state callbacks and only state callbacks have arity 5
+%% and that is intended.
+-callback state_name(
+	    event_type(),
+	    EventContent :: term(),
+	    Data :: data()) ->
+    state_function_result().
+%%
+%% State callback for callback_mode() =:= handle_event_function.
+%%
+%% Note that state callbacks and only state callbacks have arity 5
+%% and that is intended.
+-callback handle_event(
+	    event_type(),
+	    EventContent :: term(),
+	    State :: state(), % Current state
+	    Data :: data()) ->
+    handle_event_result().
+
+%% Clean up before the server terminates.
+-callback terminate(
+	    Reason :: 'normal' | 'shutdown' | {'shutdown', term()}
+		    | term(),
+	    State :: state(),
+	    Data :: data()) ->
+    any().
+
+%% Note that the new code can expect to get an OldState from
+%% the old code version not only in code_change/4 but in the first
+%% state callback function called thereafter
+-callback code_change(
+	    OldVsn :: term() | {'down', term()},
+	    OldState :: state(),
+	    OldData :: data(),
+	    Extra :: term()) ->
+    {NewCallbackMode :: callback_mode(),
+     NewState :: state(),
+     NewData :: data()}.
+
+%% Format the callback module state in some sensible that is
+%% often condensed way.  For StatusOption =:= 'normal' the perferred
+%% return term is [{data,[{"State",FormattedState}]}], and for
+%% StatusOption =:= 'terminate' it is just FormattedState.
+-callback format_status(
+	    StatusOption,
+	    [ [{Key :: term(), Value :: term()}] |
+	      state() |
+	      data()]) ->
+    Status :: term() when
+      StatusOption :: 'normal' | 'terminate'.
+
+-optional_callbacks(
+   [init/1, % One may use enter_loop/5,6,7 instead
+    format_status/2, % Has got a default implementation
+    %%
+    state_name/3, % Example for callback_mode =:= state_functions:
+    %% there has to be a StateName/5 callback function for every StateName.
+    %%
+    handle_event/4]). % For callback_mode =:= handle_event_function
+
+%% Type validation functions
+callback_mode(CallbackMode) ->
+    case CallbackMode of
+	state_functions ->
+	    true;
+	handle_event_function ->
+	    true;
+	_ ->
+	    false
+    end.
+%%
+from({Pid,_}) when is_pid(Pid) ->
+    true;
+from(_) ->
+    false.
+%%
+event_type({call,From}) ->
+    from(From);
+event_type(Type) ->
+    case Type of
+	cast ->
+	    true;
+	info ->
+	    true;
+	timeout ->
+	    true;
+	internal ->
+	    true;
+	_ ->
+	    false
+    end.
+
+
+
+-define(
+   STACKTRACE(),
+   try throw(ok) catch _ -> erlang:get_stacktrace() end).
+
+-define(
+   TERMINATE(Class, Reason, Debug, S, Q),
+   terminate(
+     begin Class end,
+     begin Reason end,
+     ?STACKTRACE(),
+     begin Debug end,
+     begin S end,
+     begin Q end)).
+
+%%%==========================================================================
+%%% API
+
+-type server_name() ::
+      {'global', GlobalName :: term()}
+      | {'via', RegMod :: module(), Name :: term()}
+      | {'local', atom()}.
+-type server_ref() ::
+      {'global', GlobalName :: term()}
+      | {'via', RegMod :: module(), ViaName :: term()}
+      | (LocalName :: atom())
+      | {Name :: atom(), Node :: atom()}
+      | pid().
+-type debug_opt() ::
+	{'debug',
+	 Dbgs ::
+	   ['trace' | 'log' | 'statistics' | 'debug'
+	    | {'logfile', string()}]}.
+-type start_opt() ::
+	debug_opt()
+      | {'timeout', Time :: timeout()}
+      | {'spawn_opt', [proc_lib:spawn_option()]}.
+-type start_ret() ::  {'ok', pid()} | 'ignore' | {'error', term()}.
+
+
+
+%% Start a state machine
+-spec start(
+	Module :: module(), Args :: term(), Opts :: [start_opt()]) ->
+		   start_ret().
+start(Module, Args, Opts) ->
+    gen:start(?MODULE, nolink, Module, Args, Opts).
+%%
+-spec start(
+	ServerName :: server_name(),
+	Module :: module(), Args :: term(), Opts :: [start_opt()]) ->
+		   start_ret().
+start(ServerName, Module, Args, Opts) ->
+    gen:start(?MODULE, nolink, ServerName, Module, Args, Opts).
+
+%% Start and link to a state machine
+-spec start_link(
+	Module :: module(), Args :: term(), Opts :: [start_opt()]) ->
+		   start_ret().
+start_link(Module, Args, Opts) ->
+    gen:start(?MODULE, link, Module, Args, Opts).
+%%
+-spec start_link(
+	ServerName :: server_name(),
+	Module :: module(), Args :: term(), Opts :: [start_opt()]) ->
+		   start_ret().
+start_link(ServerName, Module, Args, Opts) ->
+    gen:start(?MODULE, link, ServerName, Module, Args, Opts).
+
+%% Stop a state machine
+-spec stop(ServerRef :: server_ref()) -> ok.
+stop(ServerRef) ->
+    gen:stop(ServerRef).
+%%
+-spec stop(
+	ServerRef :: server_ref(),
+	Reason :: term(),
+	Timeout :: timeout()) -> ok.
+stop(ServerRef, Reason, Timeout) ->
+    gen:stop(ServerRef, Reason, Timeout).
+
+%% Send an event to a state machine that arrives with type 'event'
+-spec cast(ServerRef :: server_ref(), Msg :: term()) -> ok.
+cast({global,Name}, Msg) ->
+    try	global:send(Name, wrap_cast(Msg)) of
+	_ -> ok
+    catch
+	_:_ -> ok
+    end;
+cast({via,RegMod,Name}, Msg) ->
+    try	RegMod:send(Name, wrap_cast(Msg)) of
+	_ -> ok
+    catch
+	_:_ -> ok
+    end;
+cast({Name,Node} = ServerRef, Msg) when is_atom(Name), is_atom(Node) ->
+    send(ServerRef, wrap_cast(Msg));
+cast(ServerRef, Msg) when is_atom(ServerRef) ->
+    send(ServerRef, wrap_cast(Msg));
+cast(ServerRef, Msg) when is_pid(ServerRef) ->
+    send(ServerRef, wrap_cast(Msg)).
+
+%% Call a state machine (synchronous; a reply is expected) that
+%% arrives with type {call,From}
+-spec call(ServerRef :: server_ref(), Request :: term()) -> Reply :: term().
+call(ServerRef, Request) ->
+    call(ServerRef, Request, infinity).
+%%
+-spec call(
+	ServerRef :: server_ref(),
+	Request :: term(),
+	Timeout :: timeout()) ->
+		  Reply :: term().
+call(ServerRef, Request, infinity) ->
+    try gen:call(ServerRef, '$gen_call', Request, infinity) of
+	{ok,Reply} ->
+	    Reply
+    catch
+	Class:Reason ->
+	    erlang:raise(
+	      Class,
+	      {Reason,{?MODULE,call,[ServerRef,Request,infinity]}},
+	      erlang:get_stacktrace())
+    end;
+call(ServerRef, Request, Timeout) ->
+    %% Call server through proxy process to dodge any late reply
+    Ref = make_ref(),
+    Self = self(),
+    Pid = spawn(
+	    fun () ->
+		    Self !
+			try gen:call(
+			      ServerRef, '$gen_call', Request, Timeout) of
+			    Result ->
+				{Ref,Result}
+			catch Class:Reason ->
+				{Ref,Class,Reason,erlang:get_stacktrace()}
+			end
+	    end),
+    Mref = monitor(process, Pid),
+    receive
+	{Ref,Result} ->
+	    demonitor(Mref, [flush]),
+	    case Result of
+		{ok,Reply} ->
+		    Reply
+	    end;
+	{Ref,Class,Reason,Stacktrace} ->
+	    demonitor(Mref, [flush]),
+	    erlang:raise(
+	      Class,
+	      {Reason,{?MODULE,call,[ServerRef,Request,Timeout]}},
+	      Stacktrace);
+	{'DOWN',Mref,_,_,Reason} ->
+	    %% There is a theoretical possibility that the
+	    %% proxy process gets killed between try--of and !
+	    %% so this clause is in case of that
+	    exit(Reason)
+    end.
+
+%% Reply from a state machine callback to whom awaits in call/2
+-spec reply([reply_action()] | reply_action()) -> ok.
+reply({reply,From,Reply}) ->
+    reply(From, Reply);
+reply(Replies) when is_list(Replies) ->
+    replies(Replies).
+%%
+-spec reply(From :: from(), Reply :: term()) -> ok.
+reply({To,Tag}, Reply) when is_pid(To) ->
+    Msg = {Tag,Reply},
+    try To ! Msg of
+	_ ->
+	    ok
+    catch
+	_:_ -> ok
+    end.
+
+%% Instead of starting the state machine through start/3,4
+%% or start_link/3,4 turn the current process presumably
+%% started by proc_lib into a state machine using
+%% the same arguments as you would have returned from init/1
+-spec enter_loop(
+	Module :: module(), Opts :: [debug_opt()],
+	CallbackMode :: callback_mode(),
+	State :: state(), Data :: data()) ->
+			no_return().
+enter_loop(Module, Opts, CallbackMode, State, Data) ->
+    enter_loop(Module, Opts, CallbackMode, State, Data, self()).
+%%
+-spec enter_loop(
+	Module :: module(), Opts :: [debug_opt()],
+	CallbackMode :: callback_mode(),
+	State :: state(), Data :: data(),
+	Server_or_Actions ::
+	  server_name() | pid() | [action()]) ->
+			no_return().
+enter_loop(Module, Opts, CallbackMode, State, Data, Server_or_Actions) ->
+    if
+	is_list(Server_or_Actions) ->
+	    enter_loop(
+	      Module, Opts, CallbackMode, State, Data,
+	      self(), Server_or_Actions);
+	true ->
+	    enter_loop(
+	      Module, Opts, CallbackMode, State, Data,
+	      Server_or_Actions, [])
+    end.
+%%
+-spec enter_loop(
+	Module :: module(), Opts :: [debug_opt()],
+	CallbackMode :: callback_mode(),
+	State :: state(), Data :: data(),
+	Server :: server_name() | pid(),
+	Actions :: [action()] | action()) ->
+			no_return().
+enter_loop(Module, Opts, CallbackMode, State, Data, Server, Actions) ->
+    is_atom(Module) orelse error({atom,Module}),
+    callback_mode(CallbackMode) orelse error({callback_mode,CallbackMode}),
+    Parent = gen:get_parent(),
+    enter(Module, Opts, CallbackMode, State, Data, Server, Actions, Parent).
+
+%%---------------------------------------------------------------------------
+%% API helpers
+
+wrap_cast(Event) ->
+    {'$gen_cast',Event}.
+
+replies([{reply,From,Reply}|Replies]) ->
+    reply(From, Reply),
+    replies(Replies);
+replies([]) ->
+    ok.
+
+%% Might actually not send the message in case of caught exception
+send(Proc, Msg) ->
+    try erlang:send(Proc, Msg, [noconnect]) of
+	noconnect ->
+	    _ = spawn(erlang, send, [Proc,Msg]),
+	    ok;
+	ok ->
+	    ok
+    catch
+	_:_ ->
+	    ok
+    end.
+
+%% Here init_it/6 and enter_loop/5,6,7 functions converge
+enter(Module, Opts, CallbackMode, State, Data, Server, Actions, Parent) ->
+    %% The values should already have been type checked
+    Name = gen:get_proc_name(Server),
+    Debug = gen:debug_options(Name, Opts),
+    PrevState = make_ref(), % Will be discarded by loop_event_actions/9
+    NewActions =
+	if
+	    is_list(Actions) ->
+		Actions ++ [{postpone,false}];
+	    true ->
+		[Actions,{postpone,false}]
+	end,
+    S =	#{
+      callback_mode => CallbackMode,
+      module => Module,
+      name => Name,
+      state => PrevState,
+      data => Data,
+      timer => undefined,
+      postponed => [],
+      hibernate => false},
+    loop_event_actions(
+      Parent, Debug, S, [],
+      {event,undefined}, % Will be discarded thanks to {postpone,false}
+      PrevState, State, Data, NewActions).
+
+%%%==========================================================================
+%%%  gen callbacks
+
+init_it(Starter, self, ServerRef, Module, Args, Opts) ->
+    init_it(Starter, self(), ServerRef, Module, Args, Opts);
+init_it(Starter, Parent, ServerRef, Module, Args, Opts) ->
+    try Module:init(Args) of
+	Result ->
+	    init_result(Starter, Parent, ServerRef, Module, Result, Opts)
+    catch
+	Result ->
+	    init_result(Starter, Parent, ServerRef, Module, Result, Opts);
+	Class:Reason ->
+	    gen:unregister_name(ServerRef),
+	    proc_lib:init_ack(Starter, {error,Reason}),
+	    erlang:raise(Class, Reason, erlang:get_stacktrace())
+    end.
+
+%%---------------------------------------------------------------------------
+%% gen callbacks helpers
+
+init_result(Starter, Parent, ServerRef, Module, Result, Opts) ->
+    case Result of
+	{CallbackMode,State,Data} ->
+	    case callback_mode(CallbackMode) of
+		true ->
+		    proc_lib:init_ack(Starter, {ok,self()}),
+		    enter(
+		      Module, Opts, CallbackMode, State, Data,
+		      ServerRef, [], Parent);
+		false ->
+		    Error = {callback_mode,CallbackMode},
+		    proc_lib:init_ack(Starter, {error,Error}),
+		    exit(Error)
+	    end;
+	{CallbackMode,State,Data,Actions} ->
+	    case callback_mode(CallbackMode) of
+		true ->
+		    proc_lib:init_ack(Starter, {ok,self()}),
+		    enter(
+		      Module, Opts, CallbackMode, State, Data,
+		      ServerRef, Actions, Parent);
+		false ->
+		    Error = {callback_mode,CallbackMode},
+		    proc_lib:init_ack(Starter, {error,Error}),
+		    exit(Error)
+	    end;
+	{stop,Reason} ->
+	    gen:unregister_name(ServerRef),
+	    proc_lib:init_ack(Starter, {error,Reason}),
+	    exit(Reason);
+	ignore ->
+	    gen:unregister_name(ServerRef),
+	    proc_lib:init_ack(Starter, ignore),
+	    exit(normal);
+	_ ->
+	    Error = {bad_return_value,Result},
+	    proc_lib:init_ack(Starter, {error,Error}),
+	    exit(Error)
+    end.
+
+%%%==========================================================================
+%%% sys callbacks
+
+system_continue(Parent, Debug, S) ->
+    loop(Parent, Debug, S).
+
+system_terminate(Reason, _Parent, Debug, S) ->
+    ?TERMINATE(exit, Reason, Debug, S, []).
+
+system_code_change(
+  #{module := Module,
+    state := State,
+    data := Data} = S,
+  _Mod, OldVsn, Extra) ->
+    case
+	try Module:code_change(OldVsn, State, Data, Extra)
+	catch
+	    Result -> Result
+	end
+    of
+	{NewCallbackMode,NewState,NewData} ->
+	    callback_mode(NewCallbackMode) orelse
+		error({callback_mode,NewCallbackMode}),
+	    {ok,S#{state := NewState, data := NewData}};
+	{ok,_} = Error ->
+	    error({case_clause,Error});
+	Error ->
+	    Error
+    end.
+
+system_get_state(#{state := State, data := Data}) ->
+    {ok,{State,Data}}.
+
+system_replace_state(
+  StateFun,
+  #{state := State,
+    data := Data} = S) ->
+    {NewState,NewData} = Result = StateFun({State,Data}),
+    {ok,Result,S#{state := NewState, data := NewData}}.
+
+format_status(
+  Opt,
+  [PDict,SysState,Parent,Debug,
+   #{name := Name, postponed := P} = S]) ->
+    Header = gen:format_status_header("Status for state machine", Name),
+    Log = sys:get_debug(log, Debug, []),
+    [{header,Header},
+     {data,
+      [{"Status",SysState},
+       {"Parent",Parent},
+       {"Logged Events",Log},
+       {"Postponed",P}]} |
+     case format_status(Opt, PDict, S) of
+	 L when is_list(L) -> L;
+	 T -> [T]
+     end].
+
+%%---------------------------------------------------------------------------
+%% Format debug messages.  Print them as the call-back module sees
+%% them, not as the real erlang messages.  Use trace for that.
+%%---------------------------------------------------------------------------
+
+print_event(Dev, {in,Event}, #{name := Name}) ->
+    io:format(
+      Dev, "*DBG* ~p received ~s~n",
+      [Name,event_string(Event)]);
+print_event(Dev, {out,Reply,{To,_Tag}}, #{name := Name}) ->
+    io:format(
+      Dev, "*DBG* ~p sent ~p to ~p~n",
+      [Name,Reply,To]);
+print_event(Dev, {Tag,Event,NewState}, #{name := Name, state := State}) ->
+    StateString =
+	case NewState of
+	    State ->
+		io_lib:format("~p", [State]);
+	    _ ->
+		io_lib:format("~p => ~p", [State,NewState])
+	end,
+    io:format(
+      Dev, "*DBG* ~p ~w ~s in state ~s~n",
+      [Name,Tag,event_string(Event),StateString]).
+
+event_string(Event) ->
+    case Event of
+	{{call,{Pid,_Tag}},Request} ->
+	    io_lib:format("call ~p from ~w", [Request,Pid]);
+	{Tag,Content} ->
+	    io_lib:format("~w ~p", [Tag,Content])
+    end.
+
+sys_debug(Debug, S, Entry) ->
+    case Debug of
+	[] ->
+	    Debug;
+	_ ->
+	    sys:handle_debug(Debug, fun print_event/3, S, Entry)
+    end.
+
+%%%==========================================================================
+%%% Internal callbacks
+
+wakeup_from_hibernate(Parent, Debug, S) ->
+    %% It is a new message that woke us up so we have to receive it now
+    loop_receive(Parent, Debug, S).
+
+%%%==========================================================================
+%%% State Machine engine implementation of proc_lib/gen server
+
+%% Server loop, consists of all loop* functions
+%% and some detours through sys and proc_lib
+
+%% Entry point for system_continue/3
+loop(Parent, Debug, #{hibernate := Hib} = S) ->
+    case Hib of
+	true ->
+	    %% Does not return but restarts process at
+	    %% wakeup_from_hibernate/3 that jumps to loop_receive/3
+	    proc_lib:hibernate(
+	      ?MODULE, wakeup_from_hibernate, [Parent,Debug,S]),
+	    error(
+	      {should_not_have_arrived_here_but_instead_in,
+	       {wakeup_from_hibernate,3}});
+	false ->
+	    loop_receive(Parent, Debug, S)
+    end.
+
+%% Entry point for wakeup_from_hibernate/3
+loop_receive(Parent, Debug, #{timer := Timer} = S) ->
+    receive
+	Msg ->
+	    case Msg of
+		{system,Pid,Req} ->
+		    #{hibernate := Hibernate} = S,
+		    %% Does not return but tail recursively calls
+		    %% system_continue/3 that jumps to loop/3
+		    sys:handle_system_msg(
+		      Req, Pid, Parent, ?MODULE, Debug, S, Hibernate);
+		{'EXIT',Parent,Reason} = EXIT ->
+		    %% EXIT is not a 2-tuple and therefore
+		    %% not an event and has no event_type(),
+		    %% but this will stand out in the crash report...
+		    ?TERMINATE(exit, Reason, Debug, S, [EXIT]);
+		{timeout,Timer,Content} when Timer =/= undefined ->
+		    loop_event(
+		      Parent, Debug, S, {timeout,Content});
+		_ ->
+		    %% Cancel Timer if running
+		    case Timer of
+			undefined ->
+			    ok;
+			_ ->
+			    case erlang:cancel_timer(Timer) of
+				TimeLeft when is_integer(TimeLeft) ->
+				    ok;
+				false ->
+				    receive
+					{timeout,Timer,_} ->
+					    ok
+				    after 0 ->
+					    ok
+				    end
+			    end
+		    end,
+		    Event =
+			case Msg of
+			    {'$gen_call',From,Request} ->
+				{{call,From},Request};
+			    {'$gen_cast',E} ->
+				{cast,E};
+			    _ ->
+				{info,Msg}
+			end,
+		    loop_event(Parent, Debug, S, Event)
+	    end
+    end.
+
+loop_event(Parent, Debug, S, Event) ->
+    %% The timer field in S is now invalid and ignored
+    %% until we get back to loop/3
+    NewDebug = sys_debug(Debug, S, {in,Event}),
+    %% Here the queue of not yet processed events is created
+    loop_events(Parent, NewDebug, S, [Event]).
+
+%% Process first the event queue, or if it is empty
+%% loop back to receive a new event
+loop_events(Parent, Debug, S, []) ->
+    loop(Parent, Debug, S);
+loop_events(
+  Parent, Debug,
+  #{callback_mode := CallbackMode,
+    module := Module,
+    state := State,
+    data := Data} = S,
+  [{Type,Content} = Event|Events] = Q) ->
+    try
+	case CallbackMode of
+	    state_functions ->
+		Module:State(Type, Content, Data);
+	    handle_event_function ->
+		Module:handle_event(Type, Content, State, Data)
+	end of
+	Result ->
+	    loop_event_result(
+	      Parent, Debug, S, Events, Event, Result)
+    catch
+	Result ->
+	    loop_event_result(
+	      Parent, Debug, S, Events, Event, Result);
+	error:badarg when CallbackMode =:= state_functions ->
+	    case erlang:get_stacktrace() of
+		[{erlang,apply,[Module,State,_],_}|Stacktrace] ->
+		    Args = [Type,Content,Data],
+		    terminate(
+		      error,
+		      {undef_state_function,{Module,State,Args}},
+		      Stacktrace,
+		      Debug, S, Q);
+		Stacktrace ->
+		    terminate(error, badarg, Stacktrace, Debug, S, Q)
+	    end;
+	error:undef ->
+	    %% Process an undef to check for the simple mistake
+	    %% of calling a nonexistent state function
+	    case erlang:get_stacktrace() of
+		[{Module,State,
+		  [Type,Content,Data]=Args,
+		  _}
+		 |Stacktrace]
+		when CallbackMode =:= state_functions ->
+		    terminate(
+		      error,
+		      {undef_state_function,{Module,State,Args}},
+		      Stacktrace,
+		      Debug, S, Q);
+		[{Module,handle_event,
+		  [Type,Content,State,Data]=Args,
+		  _}
+		 |Stacktrace]
+		when CallbackMode =:= handle_event_function ->
+		    terminate(
+		      error,
+		      {undef_state_function,
+		       {Module,handle_event,Args}},
+		      Stacktrace,
+		      Debug, S, Q);
+		Stacktrace ->
+		    terminate(error, undef, Stacktrace, Debug, S, Q)
+	    end;
+	Class:Reason ->
+	    Stacktrace = erlang:get_stacktrace(),
+	    terminate(Class, Reason, Stacktrace, Debug, S, Q)
+    end.
+
+%% Interpret all callback return variants
+loop_event_result(
+  Parent, Debug,
+  #{state := State, data := Data} = S,
+  Events, Event, Result) ->
+    case Result of
+	stop ->
+	    ?TERMINATE(exit, normal, Debug, S, [Event|Events]);
+	{stop,Reason} ->
+	    ?TERMINATE(exit, Reason, Debug, S, [Event|Events]);
+	{stop,Reason,NewData} ->
+	    NewS = S#{data := NewData},
+	    Q = [Event|Events],
+	    ?TERMINATE(exit, Reason, Debug, NewS, Q);
+	{stop_and_reply,Reason,Replies} ->
+	    Q = [Event|Events],
+	    [Class,NewReason,Stacktrace,NewDebug] =
+		reply_then_terminate(
+		  exit, Reason, ?STACKTRACE(), Debug, S, Q, Replies),
+	    %% Since we got back here Replies was bad
+	    terminate(Class, NewReason, Stacktrace, NewDebug, S, Q);
+	{stop_and_reply,Reason,Replies,NewData} ->
+	    NewS = S#{data := NewData},
+	    Q = [Event|Events],
+	    [Class,NewReason,Stacktrace,NewDebug] =
+		reply_then_terminate(
+		  exit, Reason, ?STACKTRACE(), Debug, NewS, Q, Replies),
+	    %% Since we got back here Replies was bad
+	    terminate(Class, NewReason, Stacktrace, NewDebug, NewS, Q);
+	{next_state,NextState,NewData} ->
+	    loop_event_actions(
+	      Parent, Debug, S, Events, Event,
+	      State, NextState, NewData, []);
+	{next_state,NextState,NewData,Actions} ->
+	    loop_event_actions(
+	      Parent, Debug, S, Events, Event,
+	      State, NextState, NewData, Actions);
+	{keep_state,NewData} ->
+	    loop_event_actions(
+	      Parent, Debug, S, Events, Event,
+	      State, State, NewData, []);
+	{keep_state,NewData,Actions} ->
+	    loop_event_actions(
+	      Parent, Debug, S, Events, Event,
+	      State, State, NewData, Actions);
+	keep_state_and_data ->
+	    loop_event_actions(
+	      Parent, Debug, S, Events, Event,
+	      State, State, Data, []);
+	{keep_state_and_data,Actions} ->
+	    loop_event_actions(
+	      Parent, Debug, S, Events, Event,
+	      State, State, Data, Actions);
+	_ ->
+	    ?TERMINATE(
+	      error, {bad_return_value,Result}, Debug, S, [Event|Events])
+    end.
+
+loop_event_actions(
+  Parent, Debug, S, Events, Event, State, NextState, NewData, Actions) ->
+    Postpone = false, % Shall we postpone this event, true or false
+    Hibernate = false,
+    Timeout = undefined,
+    NextEvents = [],
+    loop_event_actions(
+      Parent, Debug, S, Events, Event, State, NextState, NewData,
+      if
+	  is_list(Actions) ->
+	      Actions;
+	  true ->
+	      [Actions]
+      end,
+      Postpone, Hibernate, Timeout, NextEvents).
+%%
+%% Process all action()s
+loop_event_actions(
+  Parent, Debug, S, Events, Event,
+  State, NextState, NewData, [Action|Actions],
+  Postpone, Hibernate, Timeout, NextEvents) ->
+    case Action of
+	%% Actual actions
+	{reply,From,Reply} ->
+	    case from(From) of
+		true ->
+		    NewDebug = do_reply(Debug, S, From, Reply),
+		    loop_event_actions(
+		      Parent, NewDebug, S, Events, Event,
+		      State, NextState, NewData, Actions,
+		      Postpone, Hibernate, Timeout, NextEvents);
+		false ->
+		    ?TERMINATE(
+		       error, {bad_action,Action}, Debug, S, [Event|Events])
+	    end;
+	{next_event,Type,Content} ->
+	    case event_type(Type) of
+		true ->
+		    loop_event_actions(
+		      Parent, Debug, S, Events, Event,
+		      State, NextState, NewData, Actions,
+		      Postpone, Hibernate, Timeout,
+		      [{Type,Content}|NextEvents]);
+		false ->
+		    ?TERMINATE(
+		       error, {bad_action,Action}, Debug, S, [Event|Events])
+	    end;
+	%% Actions that set options
+	{postpone,NewPostpone} when is_boolean(NewPostpone) ->
+	    loop_event_actions(
+	      Parent, Debug, S, Events, Event,
+	      State, NextState, NewData, Actions,
+	      NewPostpone, Hibernate, Timeout, NextEvents);
+	{postpone,_} ->
+	    ?TERMINATE(
+	       error, {bad_action,Action}, Debug, S, [Event|Events]);
+	postpone ->
+	    loop_event_actions(
+	      Parent, Debug, S, Events, Event,
+	      State, NextState, NewData, Actions,
+	      true, Hibernate, Timeout, NextEvents);
+	{hibernate,NewHibernate} when is_boolean(NewHibernate) ->
+	    loop_event_actions(
+	      Parent, Debug, S, Events, Event,
+	      State, NextState, NewData, Actions,
+	      Postpone, NewHibernate, Timeout, NextEvents);
+	{hibernate,_} ->
+	    ?TERMINATE(
+	       error, {bad_action,Action}, Debug, S, [Event|Events]);
+	hibernate ->
+	    loop_event_actions(
+	      Parent, Debug, S, Events, Event,
+	      State, NextState, NewData, Actions,
+	      Postpone, true, Timeout, NextEvents);
+	{timeout,infinity,_} -> % Clear timer - it will never trigger
+	    loop_event_actions(
+	      Parent, Debug, S, Events, Event,
+	      State, NextState, NewData, Actions,
+	      Postpone, Hibernate, undefined, NextEvents);
+	{timeout,Time,_} = NewTimeout when is_integer(Time), Time >= 0 ->
+	    loop_event_actions(
+	      Parent, Debug, S, Events, Event,
+	      State, NextState, NewData, Actions,
+	      Postpone, Hibernate, NewTimeout, NextEvents);
+	{timeout,_,_} ->
+	    ?TERMINATE(
+	       error, {bad_action,Action}, Debug, S, [Event|Events]);
+	infinity -> % Clear timer - it will never trigger
+	    loop_event_actions(
+	      Parent, Debug, S, Events, Event,
+	      State, NextState, NewData, Actions,
+	      Postpone, Hibernate, undefined, NextEvents);
+	Time when is_integer(Time), Time >= 0 ->
+	    NewTimeout = {timeout,Time,Time},
+	    loop_event_actions(
+	      Parent, Debug, S, Events, Event,
+	      State, NextState, NewData, Actions,
+	      Postpone, Hibernate, NewTimeout, NextEvents);
+	_ ->
+	    ?TERMINATE(
+	       error, {bad_action,Action}, Debug, S, [Event|Events])
+    end;
+%%
+%% End of actions list
+loop_event_actions(
+  Parent, Debug, #{postponed := P0} = S, Events, Event,
+  State, NextState, NewData, [],
+  Postpone, Hibernate, Timeout, NextEvents) ->
+    %%
+    %% All options have been collected and next_events are buffered.
+    %% Do the actual state transition.
+    %%
+    P1 = % Move current event to postponed if Postpone
+	case Postpone of
+	    true ->
+		[Event|P0];
+	    false ->
+		P0
+	end,
+    {Q2,P} = % Move all postponed events to queue if state change
+	if
+	    NextState =:= State ->
+		{Events,P1};
+	    true ->
+		{lists:reverse(P1, Events),[]}
+	end,
+    %% Place next events first in queue
+    Q3 = lists:reverse(NextEvents, Q2),
+    %%
+    NewDebug =
+	sys_debug(
+	  Debug, S,
+	  case Postpone of
+	      true ->
+		  {postpone,Event,NextState};
+	      false ->
+		  {consume,Event,NextState}
+	  end),
+    %% Have a peek on the event queue so we can avoid starting
+    %% the state timer unless we have to
+    {Q,Timer} =
+	case Timeout of
+	    undefined ->
+		%% No state timeout has been requested
+		{Q3,undefined};
+	    {timeout,Time,EventContent} ->
+		%% A state timeout has been requested
+		case Q3 of
+		    [] when Time =:= 0 ->
+			%% Immediate timeout - simulate it
+			%% so we do not get the timeout message
+			%% after any received event
+			{[{timeout,EventContent}],undefined};
+		    [] ->
+			%% Actually start a timer
+			{Q3,erlang:start_timer(Time, self(), EventContent)};
+		    _ ->
+			%% Do not start a timer since any queued
+			%% event cancels the state timer so we pretend
+			%% that the timer has been started and cancelled
+			{Q3,undefined}
+		end
+	end,
+    %% Loop to top of event queue loop; process next event
+    loop_events(
+      Parent, NewDebug,
+      S#{
+	state := NextState,
+	data := NewData,
+	timer := Timer,
+	postponed := P,
+	hibernate := Hibernate},
+      Q).
+
+%%---------------------------------------------------------------------------
+%% Server helpers
+
+reply_then_terminate(Class, Reason, Stacktrace, Debug, S, Q, Replies) ->
+    if
+	is_list(Replies) ->
+	    do_reply_then_terminate(
+	      Class, Reason, Stacktrace, Debug, S, Q, Replies);
+	true ->
+	    do_reply_then_terminate(
+	      Class, Reason, Stacktrace, Debug, S, Q, [Replies])
+    end.
+%%
+do_reply_then_terminate(Class, Reason, Stacktrace, Debug, S, Q, []) ->
+    terminate(Class, Reason, Stacktrace, Debug, S, Q);
+do_reply_then_terminate(
+  Class, Reason, Stacktrace, Debug, S, Q, [R|Rs]) ->
+    case R of
+	{reply,{_To,_Tag}=From,Reply} ->
+	    NewDebug = do_reply(Debug, S, From, Reply),
+	    do_reply_then_terminate(
+	      Class, Reason, Stacktrace, NewDebug, S, Q, Rs);
+	_ ->
+	    [error,{bad_action,R},?STACKTRACE(),Debug]
+    end.
+
+do_reply(Debug, S, From, Reply) ->
+    reply(From, Reply),
+    sys_debug(Debug, S, {out,Reply,From}).
+
+
+terminate(
+  Class, Reason, Stacktrace, Debug,
+  #{module := Module,
+    state := State, data := Data} = S,
+  Q) ->
+    try Module:terminate(Reason, State, Data) of
+	_ -> ok
+    catch
+	_ -> ok;
+	C:R ->
+	    ST = erlang:get_stacktrace(),
+	    error_info(
+	      C, R, ST, Debug, S, Q,
+	      format_status(terminate, get(), S)),
+	    erlang:raise(C, R, ST)
+    end,
+    case Reason of
+	normal -> ok;
+	shutdown -> ok;
+	{shutdown,_} -> ok;
+	_ ->
+	    error_info(
+	      Class, Reason, Stacktrace, Debug, S, Q,
+	      format_status(terminate, get(), S))
+    end,
+    case Stacktrace of
+	[] ->
+	    erlang:Class(Reason);
+	_ ->
+	    erlang:raise(Class, Reason, Stacktrace)
+    end.
+
+error_info(
+  Class, Reason, Stacktrace, Debug,
+  #{name := Name, callback_mode := CallbackMode,
+    state := State, postponed := P},
+  Q, FmtData) ->
+    {FixedReason,FixedStacktrace} =
+	case Stacktrace of
+	    [{M,F,Args,_}|ST]
+	      when Class =:= error, Reason =:= undef ->
+		case code:is_loaded(M) of
+		    false ->
+			{{'module could not be loaded',M},ST};
+		    _ ->
+			Arity =
+			    if
+				is_list(Args) ->
+				    length(Args);
+				is_integer(Args) ->
+				    Args
+			    end,
+			case erlang:function_exported(M, F, Arity) of
+			    true ->
+				{Reason,Stacktrace};
+			    false ->
+				{{'function not exported',{M,F,Arity}},
+				 ST}
+			end
+		end;
+	    _ -> {Reason,Stacktrace}
+	end,
+    error_logger:format(
+      "** State machine ~p terminating~n" ++
+	  case Q of
+	      [] ->
+		  "";
+	      _ ->
+		  "** Last event = ~p~n"
+	  end ++
+	  "** When Server state  = ~p~n" ++
+	  "** Reason for termination = ~w:~p~n" ++
+	  "** State = ~p~n" ++
+	  "** Callback mode = ~p~n" ++
+	  "** Queued/Postponed = ~w/~w~n" ++
+	  case FixedStacktrace of
+	      [] ->
+		  "";
+	      _ ->
+		  "** Stacktrace =~n"
+		      "**  ~p~n"
+	  end,
+      [Name |
+       case Q of
+	   [] ->
+	       [];
+	   [Event|_] ->
+	       [Event]
+       end] ++
+	  [FmtData,Class,FixedReason,
+	   State,CallbackMode,length(Q),length(P)] ++
+	  case FixedStacktrace of
+	      [] ->
+		  [];
+	      _ ->
+		  [FixedStacktrace]
+	  end),
+    sys:print_log(Debug),
+    ok.
+
+
+%% Call Module:format_status/2 or return a default value
+format_status(
+  Opt, PDict,
+  #{module := Module, state := State, data := Data}) ->
+    case erlang:function_exported(Module, format_status, 2) of
+	true ->
+	    try Module:format_status(Opt, [PDict,State,Data])
+	    catch
+		Result -> Result;
+		_:_ ->
+		    format_status_default(
+		      Opt, State,
+		      "Module:format_status/2 crashed")
+	    end;
+	false ->
+	    format_status_default(Opt, State, Data)
+    end.
+
+%% The default Module:format_status/2
+format_status_default(Opt, State, Data) ->
+    SSD = {State,Data},
+    case Opt of
+	terminate ->
+	    SSD;
+	_ ->
+	    [{data,[{"State",SSD}]}]
+    end.
diff --git a/lib/stdlib/src/proc_lib.erl b/lib/stdlib/src/proc_lib.erl
index c47d30c8b8..3f79ed0f87 100644
--- a/lib/stdlib/src/proc_lib.erl
+++ b/lib/stdlib/src/proc_lib.erl
@@ -472,13 +472,15 @@ trans_init(gen,init_it,[gen_server,_,_,supervisor_bridge,[Module|_],_]) ->
     {supervisor_bridge,Module,1};
 trans_init(gen,init_it,[gen_server,_,_,_,supervisor_bridge,[Module|_],_]) ->
     {supervisor_bridge,Module,1};
-trans_init(gen,init_it,[gen_server,_,_,Module,_,_]) ->
+trans_init(gen,init_it,[GenMod,_,_,Module,_,_])
+  when GenMod =:= gen_server;
+       GenMod =:= gen_statem;
+       GenMod =:= gen_fsm ->
     {Module,init,1};
-trans_init(gen,init_it,[gen_server,_,_,_,Module|_]) ->
-    {Module,init,1};
-trans_init(gen,init_it,[gen_fsm,_,_,Module,_,_]) ->
-    {Module,init,1};
-trans_init(gen,init_it,[gen_fsm,_,_,_,Module|_]) ->
+trans_init(gen,init_it,[GenMod,_,_,_,Module|_])
+  when GenMod =:= gen_server;
+       GenMod =:= gen_statem;
+       GenMod =:= gen_fsm ->
     {Module,init,1};
 trans_init(gen,init_it,[gen_event|_]) ->
     {gen_event,init_it,6};
diff --git a/lib/stdlib/src/stdlib.app.src b/lib/stdlib/src/stdlib.app.src
index e3950ee795..32bcdc4f2a 100644
--- a/lib/stdlib/src/stdlib.app.src
+++ b/lib/stdlib/src/stdlib.app.src
@@ -65,6 +65,7 @@
 	     gen_event,
 	     gen_fsm,
 	     gen_server,
+	     gen_statem,
 	     io,
 	     io_lib,
 	     io_lib_format,
diff --git a/lib/stdlib/test/Makefile b/lib/stdlib/test/Makefile
index 287f63b2be..28c35aed55 100644
--- a/lib/stdlib/test/Makefile
+++ b/lib/stdlib/test/Makefile
@@ -46,6 +46,7 @@ MODULES= \
 	gen_event_SUITE \
 	gen_fsm_SUITE \
 	gen_server_SUITE \
+	gen_statem_SUITE \
 	id_transform_SUITE \
 	io_SUITE \
 	io_proto_SUITE \
diff --git a/lib/stdlib/test/error_logger_forwarder.erl b/lib/stdlib/test/error_logger_forwarder.erl
index 0aecd41ba9..6bbf180585 100644
--- a/lib/stdlib/test/error_logger_forwarder.erl
+++ b/lib/stdlib/test/error_logger_forwarder.erl
@@ -20,7 +20,7 @@
 -module(error_logger_forwarder).
 
 %% API.
--export([register/0]).
+-export([register/0, unregister/0]).
 
 %% Internal export for error_logger.
 -export([init/1,
@@ -33,6 +33,10 @@
 register() ->
     error_logger:add_report_handler(?MODULE, self()).
 
+unregister() ->
+    Self = self(),
+    Self = error_logger:delete_report_handler(?MODULE).
+
 init(Tester) ->
     {ok,Tester}.
     
diff --git a/lib/stdlib/test/gen_statem_SUITE.erl b/lib/stdlib/test/gen_statem_SUITE.erl
new file mode 100644
index 0000000000..3deb5fd986
--- /dev/null
+++ b/lib/stdlib/test/gen_statem_SUITE.erl
@@ -0,0 +1,1578 @@
+%%
+%% %CopyrightBegin%
+%%
+%% Copyright Ericsson AB 2016. All Rights Reserved.
+%%
+%% Licensed under the Apache License, Version 2.0 (the "License");
+%% you may not use this file except in compliance with the License.
+%% You may obtain a copy of the License at
+%%
+%%     http://www.apache.org/licenses/LICENSE-2.0
+%%
+%% Unless required by applicable law or agreed to in writing, software
+%% distributed under the License is distributed on an "AS IS" BASIS,
+%% WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+%% See the License for the specific language governing permissions and
+%% limitations under the License.
+%%
+%% %CopyrightEnd%
+%%
+-module(gen_statem_SUITE).
+
+-include_lib("common_test/include/ct.hrl").
+
+-compile(export_all).
+-behaviour(gen_statem).
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+
+suite() ->
+    [{ct_hooks,[ts_install_cth]},
+     {timetrap,{minutes,1}}].
+
+all() ->
+    [{group, start},
+     {group, start_handle_event},
+     {group, stop},
+     {group, stop_handle_event},
+     {group, abnormal},
+     {group, abnormal_handle_event},
+     shutdown, stop_and_reply, event_order,
+     {group, sys},
+     hibernate, enter_loop].
+
+groups() ->
+    [{start, [],
+      [start1, start2, start3, start4, start5, start6, start7,
+       start8, start9, start10, start11, start12, next_events]},
+     {start_handle_event, [],
+      [start1, start2, start3, start4, start5, start6, start7,
+       start8, start9, start10, start11, start12, next_events]},
+     {stop, [],
+      [stop1, stop2, stop3, stop4, stop5, stop6, stop7, stop8, stop9, stop10]},
+     {stop_handle_event, [],
+      [stop1, stop2, stop3, stop4, stop5, stop6, stop7, stop8, stop9, stop10]},
+     {abnormal, [], [abnormal1, abnormal2]},
+     {abnormal_handle_event, [], [abnormal1, abnormal2]},
+     {sys, [],
+      [sys1, code_change,
+       call_format_status,
+       error_format_status, terminate_crash_format,
+       get_state, replace_state]},
+     {sys_handle_event, [],
+      [sys1,
+       call_format_status,
+       error_format_status, terminate_crash_format,
+       get_state, replace_state]}].
+
+init_per_suite(Config) ->
+    Config.
+
+end_per_suite(_Config) ->
+    ok.
+
+init_per_group(GroupName, Config)
+  when GroupName =:= start_handle_event;
+       GroupName =:= stop_handle_event;
+       GroupName =:= abnormal_handle_event;
+       GroupName =:= sys_handle_event ->
+    [{callback_mode,handle_event_function}|Config];
+init_per_group(_GroupName, Config) ->
+    Config.
+
+end_per_group(_GroupName, Config) ->
+    Config.
+
+init_per_testcase(_CaseName, Config) ->
+    flush(),
+%%%    dbg:tracer(),
+%%%    dbg:p(all, c),
+%%%    dbg:tpl(gen_statem, cx),
+%%%    dbg:tpl(proc_lib, cx),
+%%%    dbg:tpl(gen, cx),
+%%%    dbg:tpl(sys, cx),
+    Config.
+
+end_per_testcase(_CaseName, Config) ->
+%%%    dbg:stop(),
+    Config.
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+-define(EXPECT_FAILURE(Code, Reason),
+	try begin Code end of
+	    Reason ->
+		ct:fail({unexpected,Reason})
+	catch
+	    error:Reason -> Reason;
+	    exit:Reason -> Reason
+	end).
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+
+%% anonymous
+start1(Config) ->
+    %%OldFl = process_flag(trap_exit, true),
+
+    {ok,Pid0} = gen_statem:start_link(?MODULE, start_arg(Config, []), []),
+    ok = do_func_test(Pid0),
+    ok = do_sync_func_test(Pid0),
+    stop_it(Pid0),
+%%    stopped = gen_statem:call(Pid0, stop),
+%%    timeout =
+%%	?EXPECT_FAILURE(gen_statem:call(Pid0, hej), Reason),
+
+    %%process_flag(trap_exit, OldFl),
+    ok = verify_empty_msgq().
+
+%% anonymous w. shutdown
+start2(Config) ->
+    %% Dont link when shutdown
+    {ok,Pid0} = gen_statem:start(?MODULE, start_arg(Config, []), []),
+    ok = do_func_test(Pid0),
+    ok = do_sync_func_test(Pid0),
+    stopped = gen_statem:call(Pid0, {stop,shutdown}),
+    check_stopped(Pid0),
+    ok = verify_empty_msgq().
+
+%% anonymous with timeout
+start3(Config) ->
+    %%OldFl = process_flag(trap_exit, true),
+
+    {ok,Pid0} =
+	gen_statem:start(?MODULE, start_arg(Config, []), [{timeout,5}]),
+    ok = do_func_test(Pid0),
+    ok = do_sync_func_test(Pid0),
+    stop_it(Pid0),
+
+    {error,timeout} =
+	gen_statem:start(
+	  ?MODULE, start_arg(Config, sleep), [{timeout,5}]),
+
+    %%process_flag(trap_exit, OldFl),
+    ok = verify_empty_msgq().
+
+%% anonymous with ignore
+start4(Config) ->
+    OldFl = process_flag(trap_exit, true),
+
+    ignore = gen_statem:start(?MODULE, start_arg(Config, ignore), []),
+
+    process_flag(trap_exit, OldFl),
+    ok = verify_empty_msgq().
+
+%% anonymous with stop
+start5(Config) ->
+    OldFl = process_flag(trap_exit, true),
+
+    {error,stopped} = gen_statem:start(?MODULE, start_arg(Config, stop), []),
+
+    process_flag(trap_exit, OldFl),
+    ok = verify_empty_msgq().
+
+%% anonymous linked
+start6(Config) ->
+    {ok,Pid} = gen_statem:start_link(?MODULE, start_arg(Config, []), []),
+    ok = do_func_test(Pid),
+    ok = do_sync_func_test(Pid),
+    stop_it(Pid),
+
+    ok = verify_empty_msgq().
+
+%% global register linked
+start7(Config) ->
+    STM = {global,my_stm},
+
+    {ok,Pid} =
+	gen_statem:start_link(STM, ?MODULE, start_arg(Config, []), []),
+    {error,{already_started,Pid}} =
+	gen_statem:start_link(STM, ?MODULE, start_arg(Config, []), []),
+    {error,{already_started,Pid}} =
+	gen_statem:start(STM, ?MODULE, start_arg(Config, []), []),
+
+    ok = do_func_test(Pid),
+    ok = do_sync_func_test(Pid),
+    ok = do_func_test(STM),
+    ok = do_sync_func_test(STM),
+    stop_it(STM),
+
+    ok = verify_empty_msgq().
+
+
+%% local register
+start8(Config) ->
+    %%OldFl = process_flag(trap_exit, true),
+    Name = my_stm,
+    STM = {local,Name},
+
+    {ok,Pid} =
+	gen_statem:start(STM, ?MODULE, start_arg(Config, []), []),
+    {error,{already_started,Pid}} =
+	gen_statem:start(STM, ?MODULE, start_arg(Config, []), []),
+
+    ok = do_func_test(Pid),
+    ok = do_sync_func_test(Pid),
+    ok = do_func_test(Name),
+    ok = do_sync_func_test(Name),
+    stop_it(Pid),
+
+    %%process_flag(trap_exit, OldFl),
+    ok = verify_empty_msgq().
+
+%% local register linked
+start9(Config) ->
+    %%OldFl = process_flag(trap_exit, true),
+    Name = my_stm,
+    STM = {local,Name},
+
+    {ok,Pid} =
+	gen_statem:start_link(STM, ?MODULE, start_arg(Config, []), []),
+    {error,{already_started,Pid}} =
+	gen_statem:start(STM, ?MODULE, start_arg(Config, []), []),
+
+    ok = do_func_test(Pid),
+    ok = do_sync_func_test(Pid),
+    ok = do_func_test(Name),
+    ok = do_sync_func_test(Name),
+    stop_it(Pid),
+
+    %%process_flag(trap_exit, OldFl),
+    ok = verify_empty_msgq().
+
+%% global register
+start10(Config) ->
+    STM = {global,my_stm},
+
+    {ok,Pid} =
+	gen_statem:start(STM, ?MODULE, start_arg(Config, []), []),
+    {error,{already_started,Pid}} =
+	gen_statem:start(STM, ?MODULE, start_arg(Config, []), []),
+    {error,{already_started,Pid}} =
+	gen_statem:start_link(STM, ?MODULE, start_arg(Config, []), []),
+
+    ok = do_func_test(Pid),
+    ok = do_sync_func_test(Pid),
+    ok = do_func_test(STM),
+    ok = do_sync_func_test(STM),
+    stop_it(STM),
+
+    ok = verify_empty_msgq().
+
+%% Stop registered processes
+start11(Config) ->
+    Name = my_stm,
+    LocalSTM = {local,Name},
+    GlobalSTM = {global,Name},
+
+    {ok,Pid} =
+	gen_statem:start_link(LocalSTM, ?MODULE, start_arg(Config, []), []),
+    stop_it(Pid),
+
+    {ok,_Pid1} =
+	gen_statem:start_link(LocalSTM, ?MODULE, start_arg(Config, []), []),
+    stop_it(Name),
+
+    {ok,Pid2} =
+	gen_statem:start(GlobalSTM, ?MODULE, start_arg(Config, []), []),
+    stop_it(Pid2),
+    receive after 1 -> true end,
+    Result =
+	gen_statem:start(GlobalSTM, ?MODULE, start_arg(Config, []), []),
+    ct:log("Result = ~p~n",[Result]),
+    {ok,_Pid3} = Result,
+    stop_it(GlobalSTM),
+
+    ok = verify_empty_msgq().
+
+%% Via register linked
+start12(Config) ->
+    dummy_via:reset(),
+    VIA = {via,dummy_via,my_stm},
+
+    {ok,Pid} =
+	gen_statem:start_link(VIA, ?MODULE, start_arg(Config, []), []),
+    {error,{already_started,Pid}} =
+	gen_statem:start_link(VIA, ?MODULE, start_arg(Config, []), []),
+    {error,{already_started,Pid}} =
+	gen_statem:start(VIA, ?MODULE, start_arg(Config, []), []),
+
+    ok = do_func_test(Pid),
+    ok = do_sync_func_test(Pid),
+    ok = do_func_test(VIA),
+    ok = do_sync_func_test(VIA),
+    stop_it(VIA),
+
+    ok = verify_empty_msgq().
+
+
+%% Anonymous, reason 'normal'
+stop1(Config) ->
+    {ok,Pid} = gen_statem:start(?MODULE, start_arg(Config, []), []),
+    ok = gen_statem:stop(Pid),
+    false = erlang:is_process_alive(Pid),
+    noproc =
+	?EXPECT_FAILURE(gen_statem:stop(Pid), Reason).
+
+%% Anonymous, other reason
+stop2(Config) ->
+    {ok,Pid} = gen_statem:start(?MODULE, start_arg(Config, []), []),
+    ok = gen_statem:stop(Pid, other_reason, infinity),
+    false = erlang:is_process_alive(Pid),
+    ok.
+
+%% Anonymous, invalid timeout
+stop3(Config) ->
+    {ok,Pid} = gen_statem:start(?MODULE, start_arg(Config, []), []),
+    _ =
+	?EXPECT_FAILURE(
+	   gen_statem:stop(Pid, other_reason, invalid_timeout),
+	   Reason),
+    true = erlang:is_process_alive(Pid),
+    ok = gen_statem:stop(Pid),
+    false = erlang:is_process_alive(Pid),
+    ok.
+
+%% Registered name
+stop4(Config) ->
+    {ok,Pid} =
+	gen_statem:start(
+	  {local,to_stop},?MODULE, start_arg(Config, []), []),
+    ok = gen_statem:stop(to_stop),
+    false = erlang:is_process_alive(Pid),
+    noproc =
+	?EXPECT_FAILURE(gen_statem:stop(to_stop), Reason),
+    ok.
+
+%% Registered name and local node
+stop5(Config) ->
+    Name = to_stop,
+    {ok,Pid} =
+	gen_statem:start(
+	  {local,Name},?MODULE, start_arg(Config, []), []),
+    ok = gen_statem:stop({Name,node()}),
+    false = erlang:is_process_alive(Pid),
+    noproc =
+	?EXPECT_FAILURE(gen_statem:stop({Name,node()}), Reason),
+    ok.
+
+%% Globally registered name
+stop6(Config) ->
+    STM = {global,to_stop},
+    {ok,Pid} = gen_statem:start(STM, ?MODULE, start_arg(Config, []), []),
+    ok = gen_statem:stop(STM),
+    false = erlang:is_process_alive(Pid),
+    noproc =
+	?EXPECT_FAILURE(gen_statem:stop(STM), Reason),
+    ok.
+
+%% 'via' registered name
+stop7(Config) ->
+    VIA = {via,dummy_via,to_stop},
+    dummy_via:reset(),
+    {ok,Pid} = gen_statem:start(VIA, ?MODULE, start_arg(Config, []), []),
+    ok = gen_statem:stop(VIA),
+    false = erlang:is_process_alive(Pid),
+    noproc =
+	?EXPECT_FAILURE(gen_statem:stop(VIA), Reason),
+    ok.
+
+%% Anonymous on remote node
+stop8(Config) ->
+    Node = gen_statem_stop8,
+    {ok,NodeName} = ct_slave:start(Node),
+    Dir = filename:dirname(code:which(?MODULE)),
+    rpc:call(NodeName, code, add_path, [Dir]),
+    {ok,Pid} =
+	rpc:call(
+	  NodeName, gen_statem,start,
+	  [?MODULE,start_arg(Config, []),[]]),
+    ok = gen_statem:stop(Pid),
+    false = rpc:call(NodeName, erlang, is_process_alive, [Pid]),
+    noproc =
+	?EXPECT_FAILURE(gen_statem:stop(Pid), Reason1),
+    {ok,NodeName} = ct_slave:stop(Node),
+    {{nodedown,NodeName},{sys,terminate,_}} =
+	?EXPECT_FAILURE(gen_statem:stop(Pid), Reason2),
+    ok.
+
+%% Registered name on remote node
+stop9(Config) ->
+    Name = to_stop,
+    LocalSTM = {local,Name},
+    Node = gen_statem__stop9,
+    {ok,NodeName} = ct_slave:start(Node),
+    STM = {Name,NodeName},
+    Dir = filename:dirname(code:which(?MODULE)),
+    rpc:call(NodeName, code, add_path, [Dir]),
+    {ok,Pid} =
+	rpc:call(
+	  NodeName, gen_statem, start,
+	  [LocalSTM,?MODULE,start_arg(Config, []),[]]),
+    ok = gen_statem:stop(STM),
+    undefined = rpc:call(NodeName,erlang,whereis,[Name]),
+    false = rpc:call(NodeName,erlang,is_process_alive,[Pid]),
+    noproc =
+	?EXPECT_FAILURE(gen_statem:stop(STM), Reason1),
+    {ok,NodeName} = ct_slave:stop(Node),
+    {{nodedown,NodeName},{sys,terminate,_}} =
+	?EXPECT_FAILURE(gen_statem:stop(STM), Reason2),
+    ok.
+
+%% Globally registered name on remote node
+stop10(Config) ->
+    Node = gen_statem_stop10,
+    STM = {global,to_stop},
+    {ok,NodeName} = ct_slave:start(Node),
+    Dir = filename:dirname(code:which(?MODULE)),
+    rpc:call(NodeName,code,add_path,[Dir]),
+    {ok,Pid} =
+	rpc:call(
+	  NodeName, gen_statem, start,
+	  [STM,?MODULE,start_arg(Config, []),[]]),
+    global:sync(),
+    ok = gen_statem:stop(STM),
+    false = rpc:call(NodeName, erlang, is_process_alive, [Pid]),
+    noproc =
+	?EXPECT_FAILURE(gen_statem:stop(STM), Reason1),
+    {ok,NodeName} = ct_slave:stop(Node),
+    noproc =
+	?EXPECT_FAILURE(gen_statem:stop(STM), Reason2),
+    ok.
+
+%% Check that time outs in calls work
+abnormal1(Config) ->
+    Name = abnormal1,
+    LocalSTM = {local,Name},
+
+    {ok, _Pid} =
+	gen_statem:start(LocalSTM, ?MODULE, start_arg(Config, []), []),
+
+    %% timeout call.
+    delayed = gen_statem:call(Name, {delayed_answer,1}, 100),
+    {timeout,_} =
+	?EXPECT_FAILURE(
+	   gen_statem:call(Name, {delayed_answer,1000}, 10),
+	   Reason),
+    ok = gen_statem:stop(Name),
+    ok = verify_empty_msgq().
+
+%% Check that bad return values makes the stm crash. Note that we must
+%% trap exit since we must link to get the real bad_return_ error
+abnormal2(Config) ->
+    OldFl = process_flag(trap_exit, true),
+    {ok,Pid} = gen_statem:start_link(?MODULE, start_arg(Config, []), []),
+
+    %% bad return value in the gen_statem loop
+    {{bad_return_value,badreturn},_} =
+	?EXPECT_FAILURE(gen_statem:call(Pid, badreturn), Reason),
+    receive
+	{'EXIT',Pid,{bad_return_value,badreturn}} -> ok
+    after 5000 ->
+	    ct:fail(gen_statem_did_not_die)
+    end,
+
+    process_flag(trap_exit, OldFl),
+    ok = verify_empty_msgq().
+
+shutdown(Config) ->
+    process_flag(trap_exit, true),
+
+    {ok,Pid0} = gen_statem:start_link(?MODULE, start_arg(Config, []), []),
+    ok = do_func_test(Pid0),
+    ok = do_sync_func_test(Pid0),
+    stopped = gen_statem:call(Pid0, {stop,{shutdown,reason}}),
+    receive {'EXIT',Pid0,{shutdown,reason}} -> ok end,
+    process_flag(trap_exit, false),
+
+    {noproc,_} =
+	?EXPECT_FAILURE(gen_statem:call(Pid0, hej), Reason),
+
+    receive
+	Any ->
+	    ct:log("Unexpected: ~p", [Any]),
+	    ct:fail({unexpected,Any})
+    after 500 ->
+	    ok
+    end.
+
+
+
+stop_and_reply(_Config) ->
+    process_flag(trap_exit, true),
+
+    Machine =
+	%% Abusing the internal format of From...
+	#{init =>
+	      fun () ->
+		      {ok,start,undefined}
+	      end,
+	  start =>
+	      fun (cast, {echo,From1,Reply1}, undefined) ->
+		      {next_state,wait,{reply,From1,Reply1}}
+	      end,
+	  wait =>
+	      fun (cast, {stop_and_reply,Reason,From2,Reply2},R1) ->
+		      {stop_and_reply,Reason,
+		       [R1,{reply,From2,Reply2}]}
+	      end},
+    {ok,STM} = gen_statem:start_link(?MODULE, {map_statem,Machine}, []),
+
+    Self = self(),
+    Tag1 = make_ref(),
+    gen_statem:cast(STM, {echo,{Self,Tag1},reply1}),
+    Tag2 = make_ref(),
+    gen_statem:cast(STM, {stop_and_reply,reason,{Self,Tag2},reply2}),
+    case flush() of
+	[{Tag1,reply1},{Tag2,reply2},{'EXIT',STM,reason}] ->
+	    ok;
+	Other1 ->
+	    ct:fail({unexpected,Other1})
+    end,
+
+    {noproc,_} =
+	?EXPECT_FAILURE(gen_statem:call(STM, hej), Reason),
+    case flush() of
+	[] ->
+	    ok;
+	Other2 ->
+	    ct:fail({unexpected,Other2})
+    end.
+
+
+
+event_order(_Config) ->
+    process_flag(trap_exit, true),
+
+    Machine =
+	%% Abusing the internal format of From...
+	#{init =>
+	      fun () ->
+		      {ok,start,undefined}
+	      end,
+	  start =>
+	      fun (cast, _, _) ->
+		      {keep_state_and_data,postpone}; %% Handled in 'buffer'
+		  ({call,From}, {buffer,Pid,[Tag3,Tag4,Tag5]},
+		   undefined) ->
+		      {next_state,buffer,[],
+		       [{next_event,internal,{reply,{Pid,Tag3},ok3}},
+			{next_event,internal,{reply,{Pid,Tag4},ok4}},
+			{timeout,0,{reply,{Pid,Tag5},ok5}},
+			%% The timeout should not happen since there
+			%% are events that cancel it i.e next_event
+			%% and postponed
+			{reply,From,ok}]}
+	      end,
+	  buffer =>
+	      fun (internal, Reply, Replies) ->
+		      {keep_state,[Reply|Replies]};
+		  (timeout, Reply, Replies) ->
+		      {keep_state,[Reply|Replies]};
+		  (cast, Reply, Replies) ->
+		      {keep_state,[Reply|Replies]};
+		  ({call,From}, {stop,Reason}, Replies) ->
+		      {next_state,stop,undefined,
+		       lists:reverse(
+			 Replies,
+			 [{reply,From,ok},
+			  {next_event,internal,{stop,Reason}}])}
+	      end,
+	  stop =>
+	      fun (internal, Result, undefined) ->
+		      Result
+	      end},
+
+    {ok,STM} = gen_statem:start_link(?MODULE, {map_statem,Machine}, []),
+    Self = self(),
+    Tag1 = make_ref(),
+    gen_statem:cast(STM, {reply,{Self,Tag1},ok1}),
+    Tag2 = make_ref(),
+    gen_statem:cast(STM, {reply,{Self,Tag2},ok2}),
+    Tag3 = make_ref(),
+    Tag4 = make_ref(),
+    Tag5 = make_ref(),
+    ok = gen_statem:call(STM, {buffer,Self,[Tag3,Tag4,Tag5]}),
+    ok = gen_statem:call(STM, {stop,reason}),
+    case flush() of
+	[{Tag3,ok3},{Tag4,ok4},{Tag1,ok1},{Tag2,ok2},
+	 {'EXIT',STM,reason}] ->
+	    ok;
+	Other1 ->
+	    ct:fail({unexpected,Other1})
+    end,
+
+    {noproc,_} =
+	?EXPECT_FAILURE(gen_statem:call(STM, hej), Reason),
+    case flush() of
+	[] ->
+	    ok;
+	Other2 ->
+	    ct:fail({unexpected,Other2})
+    end.
+
+
+
+sys1(Config) ->
+    {ok,Pid} = gen_statem:start(?MODULE, start_arg(Config, []), []),
+    {status, Pid, {module,gen_statem}, _} = sys:get_status(Pid),
+    sys:suspend(Pid),
+    Parent = self(),
+    Tag = make_ref(),
+    Caller =
+	spawn(
+	  fun () ->
+		  Parent ! {Tag,gen_statem:call(Pid, hej)}
+	  end),
+    receive
+	{Tag,_} ->
+	    ct:fail(should_be_suspended)
+    after 3000 ->
+	    exit(Caller, ok)
+    end,
+
+    %% {timeout,_} =
+    %% 	?EXPECT_FAILURE(gen_statem:call(Pid, hej), Reason),
+    sys:resume(Pid),
+    stop_it(Pid).
+
+code_change(Config) ->
+    {ok,Pid} = gen_statem:start(?MODULE, start_arg(Config, []), []),
+    {idle,data} = sys:get_state(Pid),
+    sys:suspend(Pid),
+    sys:change_code(Pid, ?MODULE, old_vsn, state_functions),
+    sys:resume(Pid),
+    {idle,{old_vsn,data,state_functions}} = sys:get_state(Pid),
+    stop_it(Pid).
+
+call_format_status(Config) ->
+    {ok,Pid} = gen_statem:start(?MODULE, start_arg(Config, []), []),
+    Status = sys:get_status(Pid),
+    {status,Pid,_Mod,[_PDict,running,_,_, Data]} = Status,
+    [format_status_called|_] = lists:reverse(Data),
+    stop_it(Pid),
+
+    %% check that format_status can handle a name being an atom (pid is
+    %% already checked by the previous test)
+    {ok, Pid2} =
+	gen_statem:start(
+	  {local, gstm}, ?MODULE, start_arg(Config, []), []),
+    Status2 = sys:get_status(gstm),
+    {status,Pid2,_Mod,[_PDict2,running,_,_,Data2]} = Status2,
+    [format_status_called|_] = lists:reverse(Data2),
+    stop_it(Pid2),
+
+    %% check that format_status can handle a name being a term other than a
+    %% pid or atom
+    GlobalName1 = {global,"CallFormatStatus"},
+    {ok,Pid3} =
+	gen_statem:start(
+	  GlobalName1, ?MODULE, start_arg(Config, []), []),
+    Status3 = sys:get_status(GlobalName1),
+    {status,Pid3,_Mod,[_PDict3,running,_,_,Data3]} = Status3,
+    [format_status_called|_] = lists:reverse(Data3),
+    stop_it(Pid3),
+    GlobalName2 = {global,{name, "term"}},
+    {ok,Pid4} =
+	gen_statem:start(
+	  GlobalName2, ?MODULE, start_arg(Config, []), []),
+    Status4 = sys:get_status(GlobalName2),
+    {status,Pid4,_Mod,[_PDict4,running,_,_, Data4]} = Status4,
+    [format_status_called|_] = lists:reverse(Data4),
+    stop_it(Pid4),
+
+    %% check that format_status can handle a name being a term other than a
+    %% pid or atom
+    dummy_via:reset(),
+    ViaName1 = {via,dummy_via,"CallFormatStatus"},
+    {ok,Pid5} = gen_statem:start(ViaName1, ?MODULE, start_arg(Config, []), []),
+    Status5 = sys:get_status(ViaName1),
+    {status,Pid5,_Mod, [_PDict5,running,_,_, Data5]} = Status5,
+    [format_status_called|_] = lists:reverse(Data5),
+    stop_it(Pid5),
+    ViaName2 = {via,dummy_via,{name,"term"}},
+    {ok, Pid6} =
+	gen_statem:start(
+	  ViaName2, ?MODULE, start_arg(Config, []), []),
+    Status6 = sys:get_status(ViaName2),
+    {status,Pid6,_Mod,[_PDict6,running,_,_,Data6]} = Status6,
+    [format_status_called|_] = lists:reverse(Data6),
+    stop_it(Pid6).
+
+
+
+error_format_status(Config) ->
+    error_logger_forwarder:register(),
+    OldFl = process_flag(trap_exit, true),
+    Data = "called format_status",
+    {ok,Pid} =
+	gen_statem:start(
+	  ?MODULE, start_arg(Config, {data,Data}), []),
+    %% bad return value in the gen_statem loop
+    {{bad_return_value,badreturn},_} =
+	?EXPECT_FAILURE(gen_statem:call(Pid, badreturn), Reason),
+    receive
+	{error,_,
+	 {Pid,
+	  "** State machine"++_,
+	  [Pid,{{call,_},badreturn},
+	   {formatted,idle,Data},
+	   error,{bad_return_value,badreturn}|_]}} ->
+	    ok;
+	Other when is_tuple(Other), element(1, Other) =:= error ->
+	    error_logger_forwarder:unregister(),
+	    ct:fail({unexpected,Other})
+    after 1000 ->
+	    error_logger_forwarder:unregister(),
+	    ct:fail(timeout)
+    end,
+    process_flag(trap_exit, OldFl),
+    error_logger_forwarder:unregister(),
+    receive
+	%% Comes with SASL
+	{error_report,_,{Pid,crash_report,_}} ->
+	    ok
+    after 500 ->
+	    ok
+    end,
+    ok = verify_empty_msgq().
+
+terminate_crash_format(Config) ->
+    error_logger_forwarder:register(),
+    OldFl = process_flag(trap_exit, true),
+    Data = crash_terminate,
+    {ok,Pid} =
+	gen_statem:start(
+	  ?MODULE, start_arg(Config, {data,Data}), []),
+    stop_it(Pid),
+    Self = self(),
+    receive
+	{error,_GroupLeader,
+	 {Pid,
+	  "** State machine"++_,
+	  [Pid,
+	   {{call,{Self,_}},stop},
+	   {formatted,idle,Data},
+	   exit,{crash,terminate}|_]}} ->
+	    ok;
+	Other when is_tuple(Other), element(1, Other) =:= error ->
+	    error_logger_forwarder:unregister(),
+	    ct:fail({unexpected,Other})
+    after 1000 ->
+	    error_logger_forwarder:unregister(),
+	    ct:fail(timeout)
+    end,
+    process_flag(trap_exit, OldFl),
+    error_logger_forwarder:unregister(),
+    receive
+	%% Comes with SASL
+	{error_report,_,{Pid,crash_report,_}} ->
+	    ok
+    after 500 ->
+	    ok
+    end,
+    ok = verify_empty_msgq().
+
+
+get_state(Config) ->
+    State = self(),
+    {ok,Pid} =
+	gen_statem:start(
+	  ?MODULE, start_arg(Config, {data,State}), []),
+    {idle,State} = sys:get_state(Pid),
+    {idle,State} = sys:get_state(Pid, 5000),
+    stop_it(Pid),
+
+    %% check that get_state can handle a name being an atom (pid is
+    %% already checked by the previous test)
+    {ok,Pid2} =
+	gen_statem:start(
+	  {local,gstm}, ?MODULE, start_arg(Config, {data,State}), []),
+    {idle,State} = sys:get_state(gstm),
+    {idle,State} = sys:get_state(gstm, 5000),
+    stop_it(Pid2),
+
+    %% check that get_state works when pid is sys suspended
+    {ok,Pid3} =
+	gen_statem:start(
+	  ?MODULE, start_arg(Config, {data,State}), []),
+    {idle,State} = sys:get_state(Pid3),
+    ok = sys:suspend(Pid3),
+    {idle,State} = sys:get_state(Pid3, 5000),
+    ok = sys:resume(Pid3),
+    stop_it(Pid3),
+    ok = verify_empty_msgq().
+
+replace_state(Config) ->
+    State = self(),
+    {ok, Pid} =
+	gen_statem:start(
+	  ?MODULE, start_arg(Config, {data,State}), []),
+    {idle,State} = sys:get_state(Pid),
+    NState1 = "replaced",
+    Replace1 = fun({StateName, _}) -> {StateName,NState1} end,
+    {idle,NState1} = sys:replace_state(Pid, Replace1),
+    {idle,NState1} = sys:get_state(Pid),
+    NState2 = "replaced again",
+    Replace2 = fun({idle, _}) -> {state0,NState2} end,
+    {state0,NState2} = sys:replace_state(Pid, Replace2, 5000),
+    {state0,NState2} = sys:get_state(Pid),
+    %% verify no change in state if replace function crashes
+    Replace3 = fun(_) -> error(fail) end,
+    {callback_failed,
+     {gen_statem,system_replace_state},{error,fail}} =
+	?EXPECT_FAILURE(sys:replace_state(Pid, Replace3), Reason),
+    {state0, NState2} = sys:get_state(Pid),
+    %% verify state replaced if process sys suspended
+    ok = sys:suspend(Pid),
+    Suffix2 = " and again",
+    NState3 = NState2 ++ Suffix2,
+    Replace4 = fun({StateName, _}) -> {StateName, NState3} end,
+    {state0,NState3} = sys:replace_state(Pid, Replace4),
+    ok = sys:resume(Pid),
+    {state0,NState3} = sys:get_state(Pid, 5000),
+    stop_it(Pid),
+    ok = verify_empty_msgq().
+
+%% Hibernation
+hibernate(Config) ->
+    OldFl = process_flag(trap_exit, true),
+
+    {ok,Pid0} =
+	gen_statem:start_link(
+	  ?MODULE, start_arg(Config, hiber_now), []),
+    is_in_erlang_hibernate(Pid0),
+    stop_it(Pid0),
+    receive
+	{'EXIT',Pid0,normal} -> ok
+    after 5000 ->
+	    ct:fail(gen_statem_did_not_die)
+    end,
+
+    {ok,Pid} =
+	gen_statem:start_link(?MODULE, start_arg(Config, hiber), []),
+    true = ({current_function,{erlang,hibernate,3}} =/=
+		erlang:process_info(Pid,current_function)),
+    hibernating = gen_statem:call(Pid, hibernate_sync),
+    is_in_erlang_hibernate(Pid),
+    good_morning = gen_statem:call(Pid, wakeup_sync),
+    is_not_in_erlang_hibernate(Pid),
+    hibernating = gen_statem:call(Pid, hibernate_sync),
+    is_in_erlang_hibernate(Pid),
+    please_just_five_more = gen_statem:call(Pid, snooze_sync),
+    is_in_erlang_hibernate(Pid),
+    good_morning = gen_statem:call(Pid, wakeup_sync),
+    is_not_in_erlang_hibernate(Pid),
+    ok = gen_statem:cast(Pid, hibernate_async),
+    is_in_erlang_hibernate(Pid),
+    ok = gen_statem:cast(Pid, wakeup_async),
+    is_not_in_erlang_hibernate(Pid),
+    ok = gen_statem:cast(Pid, hibernate_async),
+    is_in_erlang_hibernate(Pid),
+    ok = gen_statem:cast(Pid, snooze_async),
+    is_in_erlang_hibernate(Pid),
+    ok = gen_statem:cast(Pid, wakeup_async),
+    is_not_in_erlang_hibernate(Pid),
+
+    Pid ! hibernate_later,
+    true =
+	({current_function,{erlang,hibernate,3}} =/=
+	     erlang:process_info(Pid, current_function)),
+    is_in_erlang_hibernate(Pid),
+
+    'alive!' = gen_statem:call(Pid, 'alive?'),
+    true =
+	({current_function,{erlang,hibernate,3}} =/=
+	     erlang:process_info(Pid, current_function)),
+    Pid ! hibernate_now,
+    is_in_erlang_hibernate(Pid),
+
+    'alive!' = gen_statem:call(Pid, 'alive?'),
+    true =
+	({current_function,{erlang,hibernate,3}} =/=
+	     erlang:process_info(Pid, current_function)),
+
+    hibernating = gen_statem:call(Pid, hibernate_sync),
+    is_in_erlang_hibernate(Pid),
+    good_morning = gen_statem:call(Pid, wakeup_sync),
+    is_not_in_erlang_hibernate(Pid),
+    hibernating = gen_statem:call(Pid, hibernate_sync),
+    is_in_erlang_hibernate(Pid),
+    please_just_five_more = gen_statem:call(Pid, snooze_sync),
+    is_in_erlang_hibernate(Pid),
+    good_morning = gen_statem:call(Pid, wakeup_sync),
+    is_not_in_erlang_hibernate(Pid),
+    ok = gen_statem:cast(Pid, hibernate_async),
+    is_in_erlang_hibernate(Pid),
+    ok  = gen_statem:cast(Pid, wakeup_async),
+    is_not_in_erlang_hibernate(Pid),
+    ok = gen_statem:cast(Pid, hibernate_async),
+    is_in_erlang_hibernate(Pid),
+    ok = gen_statem:cast(Pid, snooze_async),
+    is_in_erlang_hibernate(Pid),
+    ok = gen_statem:cast(Pid, wakeup_async),
+    is_not_in_erlang_hibernate(Pid),
+
+    hibernating = gen_statem:call(Pid, hibernate_sync),
+    is_in_erlang_hibernate(Pid),
+    sys:suspend(Pid),
+    is_in_erlang_hibernate(Pid),
+    sys:resume(Pid),
+    is_in_erlang_hibernate(Pid),
+    receive after 1000 -> ok end,
+    is_in_erlang_hibernate(Pid),
+
+    good_morning  = gen_statem:call(Pid, wakeup_sync),
+    is_not_in_erlang_hibernate(Pid),
+    stop_it(Pid),
+    process_flag(trap_exit, OldFl),
+    receive
+	{'EXIT',Pid,normal} -> ok
+    after 5000 ->
+	    ct:fail(gen_statem_did_not_die)
+    end,
+    ok = verify_empty_msgq().
+
+is_in_erlang_hibernate(Pid) ->
+    receive after 1 -> ok end,
+    is_in_erlang_hibernate_1(200, Pid).
+
+is_in_erlang_hibernate_1(0, Pid) ->
+    ct:log("~p\n", [erlang:process_info(Pid, current_function)]),
+    ct:fail(not_in_erlang_hibernate_3);
+is_in_erlang_hibernate_1(N, Pid) ->
+    {current_function,MFA} = erlang:process_info(Pid, current_function),
+    case MFA of
+	{erlang,hibernate,3} ->
+	    ok;
+	_ ->
+	    receive after 10 -> ok end,
+	    is_in_erlang_hibernate_1(N-1, Pid)
+    end.
+
+is_not_in_erlang_hibernate(Pid) ->
+    receive after 1 -> ok end,
+    is_not_in_erlang_hibernate_1(200, Pid).
+
+is_not_in_erlang_hibernate_1(0, Pid) ->
+    ct:log("~p\n", [erlang:process_info(Pid, current_function)]),
+    ct:fail(not_in_erlang_hibernate_3);
+is_not_in_erlang_hibernate_1(N, Pid) ->
+    {current_function,MFA} = erlang:process_info(Pid, current_function),
+    case MFA of
+	{erlang,hibernate,3} ->
+	    receive after 10 -> ok end,
+	    is_not_in_erlang_hibernate_1(N-1, Pid);
+	_ ->
+	    ok
+    end.
+
+
+enter_loop(_Config) ->
+    OldFlag = process_flag(trap_exit, true),
+
+    dummy_via:reset(),
+
+    %% Locally registered process + {local,Name}
+    {ok,Pid1a} =
+	proc_lib:start_link(?MODULE, enter_loop, [local,local]),
+    yes = gen_statem:call(Pid1a, 'alive?'),
+    stopped = gen_statem:call(Pid1a, stop),
+    receive
+	{'EXIT',Pid1a,normal} ->
+	    ok
+    after 5000 ->
+	    ct:fail(gen_statem_did_not_die)
+    end,
+
+    %% Unregistered process + {local,Name}
+    {ok,Pid1b} =
+	proc_lib:start_link(?MODULE, enter_loop, [anon,local]),
+    receive
+	{'EXIT',Pid1b,process_not_registered} ->
+	    ok
+    after 5000 ->
+	    ct:fail(gen_statem_did_not_die)
+    end,
+
+    %% Globally registered process + {global,Name}
+    {ok,Pid2a} =
+	proc_lib:start_link(?MODULE, enter_loop, [global,global]),
+    yes = gen_statem:call(Pid2a, 'alive?'),
+    stopped = gen_statem:call(Pid2a, stop),
+    receive
+	{'EXIT',Pid2a,normal} ->
+	    ok
+    after 5000 ->
+	    ct:fail(gen_statem_did_not_die)
+    end,
+
+    %% Unregistered process + {global,Name}
+    {ok,Pid2b} =
+	proc_lib:start_link(?MODULE, enter_loop, [anon,global]),
+    receive
+	{'EXIT',Pid2b,process_not_registered_globally} ->
+	    ok
+    after 5000 ->
+	    ct:fail(gen_statem_did_not_die)
+    end,
+
+    %% Unregistered process + no name
+    {ok,Pid3} =
+	proc_lib:start_link(?MODULE, enter_loop, [anon,anon]),
+    yes = gen_statem:call(Pid3, 'alive?'),
+    stopped = gen_statem:call(Pid3, stop),
+    receive
+	{'EXIT',Pid3,normal} ->
+	    ok
+    after 5000 ->
+	    ct:fail(gen_statem_did_not_die)
+    end,
+
+    %% Process not started using proc_lib
+    CallbackMode = state_functions,
+    Pid4 =
+	spawn_link(
+	  gen_statem, enter_loop,
+	  [?MODULE,[],CallbackMode,state0,[]]),
+    receive
+	{'EXIT',Pid4,process_was_not_started_by_proc_lib} ->
+	    ok
+    after 5000 ->
+	    ct:fail(gen_statem_did_not_die)
+    end,
+
+    %% Make sure I am the parent, ie that ordering a shutdown will
+    %% result in the process terminating with Reason==shutdown
+    {ok,Pid5} =
+	proc_lib:start_link(?MODULE, enter_loop, [anon,anon]),
+    yes = gen_statem:call(Pid5, 'alive?'),
+    exit(Pid5, shutdown),
+    receive
+	{'EXIT',Pid5,shutdown} ->
+	    ok
+    after 5000 ->
+	    ct:fail(gen_statem_did_not_die)
+    end,
+
+    %% Make sure gen_statem:enter_loop does not accept {local,Name}
+    %% when it's another process than the calling one which is
+    %% registered under that name
+    register(armitage, self()),
+    {ok,Pid6a} =
+	proc_lib:start_link(?MODULE, enter_loop, [anon,local]),
+    receive
+	{'EXIT',Pid6a,process_not_registered} ->
+	    ok
+    after 1000 ->
+	    ct:fail(gen_statem_started)
+    end,
+    unregister(armitage),
+
+    %% Make sure gen_statem:enter_loop does not accept {global,Name}
+    %% when it's another process than the calling one which is
+    %% registered under that name
+    global:register_name(armitage, self()),
+    {ok,Pid6b} =
+	proc_lib:start_link(?MODULE, enter_loop, [anon,global]),
+    receive
+	{'EXIT',Pid6b,process_not_registered_globally} ->
+	    ok
+    after 1000 ->
+	    ct:fail(gen_statem_started)
+    end,
+    global:unregister_name(armitage),
+
+    dummy_via:register_name(armitage, self()),
+    {ok,Pid6c} =
+	proc_lib:start_link(?MODULE, enter_loop, [anon,via]),
+    receive
+	{'EXIT',Pid6c,{process_not_registered_via,dummy_via}} ->
+	    ok
+    after 1000 ->
+	    ct:fail(
+	      {gen_statem_started,
+	       process_info(self(), messages)})
+    end,
+    dummy_via:unregister_name(armitage),
+
+    process_flag(trap_exit, OldFlag),
+    ok = verify_empty_msgq().
+
+enter_loop(Reg1, Reg2) ->
+    process_flag(trap_exit, true),
+    case Reg1 of
+	local -> register(armitage, self());
+	global -> global:register_name(armitage, self());
+	via -> dummy_via:register_name(armitage, self());
+	anon -> ignore
+    end,
+    proc_lib:init_ack({ok, self()}),
+    CallbackMode = state_functions,
+    case Reg2 of
+	local ->
+	    gen_statem:enter_loop(
+	      ?MODULE, [], CallbackMode, state0, [], {local,armitage});
+	global ->
+	    gen_statem:enter_loop(
+	      ?MODULE, [], CallbackMode, state0, [], {global,armitage});
+	via ->
+	    gen_statem:enter_loop(
+	      ?MODULE, [], CallbackMode, state0, [],
+	      {via, dummy_via, armitage});
+	anon ->
+	    gen_statem:enter_loop(
+	      ?MODULE, [], CallbackMode, state0, [])
+    end.
+
+
+%% Test the order for multiple {next_event,T,C}
+next_events(Config) ->
+    {ok,Pid} = gen_statem:start(?MODULE, start_arg(Config, []), []),
+    ok = gen_statem:cast(Pid, next_event),
+    {state,next_events,[]} = gen_statem:call(Pid, get),
+    ok = gen_statem:stop(Pid),
+    false = erlang:is_process_alive(Pid),
+    noproc =
+	?EXPECT_FAILURE(gen_statem:stop(Pid), Reason).
+
+
+%%
+%% Functionality check
+%%
+
+wfor(Msg) ->
+    receive
+	Msg -> ok
+    after 5000 ->
+	    error(timeout)
+    end.
+
+
+stop_it(STM) ->
+    stopped = gen_statem:call(STM, stop),
+    check_stopped(STM).
+
+
+check_stopped(STM) ->
+    Call = there_you_are,
+    {_,{gen_statem,call,[_,Call,infinity]}} =
+	?EXPECT_FAILURE(gen_statem:call(STM, Call), Reason),
+    ok.
+
+
+do_func_test(STM) ->
+    ok = gen_statem:cast(STM, {'alive?',self()}),
+    wfor(yes),
+    ok = do_connect(STM),
+    ok = gen_statem:cast(STM, {'alive?',self()}),
+    wfor(yes),
+    ?t:do_times(3, ?MODULE, do_msg, [STM]),
+    ok = gen_statem:cast(STM, {'alive?',self()}),
+    wfor(yes),
+    ok = do_disconnect(STM),
+    ok = gen_statem:cast(STM, {'alive?',self()}),
+    wfor(yes),
+    ok.
+
+
+do_connect(STM) ->
+    check_state(STM, idle),
+    gen_statem:cast(STM, {connect,self()}),
+    wfor(accept),
+    check_state(STM, wfor_conf),
+    Tag = make_ref(),
+    gen_statem:cast(STM, {ping,self(),Tag}),
+    gen_statem:cast(STM, confirm),
+    wfor({pong,Tag}),
+    check_state(STM, connected),
+    ok.
+
+do_msg(STM) ->
+    check_state(STM, connected),
+    R = make_ref(),
+    ok = gen_statem:cast(STM, {msg,self(),R}),
+    wfor({ack,R}).
+
+
+do_disconnect(STM) ->
+    ok = gen_statem:cast(STM, disconnect),
+    check_state(STM, idle).
+
+check_state(STM, State) ->
+    case gen_statem:call(STM, get) of
+	{state, State, _} -> ok
+    end.
+
+do_sync_func_test(STM) ->
+    yes = gen_statem:call(STM, 'alive?'),
+    ok = do_sync_connect(STM),
+    yes = gen_statem:call(STM, 'alive?'),
+    ?t:do_times(3, ?MODULE, do_sync_msg, [STM]),
+    yes = gen_statem:call(STM, 'alive?'),
+    ok = do_sync_disconnect(STM),
+    yes = gen_statem:call(STM, 'alive?'),
+    check_state(STM, idle),
+    ok = gen_statem:call(STM, {timeout,200}),
+    yes = gen_statem:call(STM, 'alive?'),
+    check_state(STM, idle),
+    ok.
+
+
+do_sync_connect(STM) ->
+    check_state(STM, idle),
+    accept = gen_statem:call(STM, connect),
+    check_state(STM, wfor_conf),
+    Tag = make_ref(),
+    gen_statem:cast(STM, {ping,self(),Tag}),
+    yes = gen_statem:call(STM, confirm),
+    wfor({pong,Tag}),
+    check_state(STM, connected),
+    ok.
+
+do_sync_msg(STM) ->
+    check_state(STM, connected),
+    R = make_ref(),
+    {ack,R} = gen_statem:call(STM, {msg,R}),
+    ok.
+
+do_sync_disconnect(STM) ->
+    yes = gen_statem:call(STM, disconnect),
+    check_state(STM, idle).
+
+
+verify_empty_msgq() ->
+    [] = flush(),
+    ok.
+
+start_arg(Config, Arg) ->
+    case lists:keyfind(callback_mode, 1, Config) of
+	{_,CallbackMode} ->
+	    {callback_mode,CallbackMode,Arg};
+	false ->
+	    Arg
+    end.
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%%
+%% The State Machine
+%%
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+
+init(ignore) ->
+    ignore;
+init(stop) ->
+    {stop,stopped};
+init(stop_shutdown) ->
+    {stop,shutdown};
+init(sleep) ->
+    ?t:sleep(1000),
+    {state_functions,idle,data};
+init(hiber) ->
+    {state_functions,hiber_idle,[]};
+init(hiber_now) ->
+    {state_functions,hiber_idle,[],[hibernate]};
+init({data, Data}) ->
+    {state_functions,idle,Data};
+init({callback_mode,CallbackMode,Arg}) ->
+    case init(Arg) of
+	{_,State,Data,Ops} ->
+	    {CallbackMode,State,Data,Ops};
+	{_,State,Data} ->
+	    {CallbackMode,State,Data};
+	Other ->
+	    Other
+    end;
+init({map_statem,#{init := Init}=Machine}) ->
+    case Init() of
+	{ok,State,Data,Ops} ->
+	    {handle_event_function,State,[Data|Machine],Ops};
+	{ok,State,Data} ->
+	    {handle_event_function,State,[Data|Machine]};
+	Other ->
+	    Other
+    end;
+init([]) ->
+    {state_functions,idle,data}.
+
+terminate(_, _State, crash_terminate) ->
+    exit({crash,terminate});
+terminate({From,stopped}, State, _Data) ->
+    From ! {self(),{stopped,State}},
+    ok;
+terminate(_Reason, _State, _Data) ->
+    ok.
+
+
+%% State functions
+
+idle(cast, {connect,Pid}, Data) ->
+    Pid ! accept,
+    {next_state,wfor_conf,Data,infinity}; % NoOp timeout just to test API
+idle({call,From}, connect, Data) ->
+    gen_statem:reply(From, accept),
+    {next_state,wfor_conf,Data,infinity}; % NoOp timeout just to test API
+idle(cast, badreturn, _Data) ->
+    badreturn;
+idle({call,_From}, badreturn, _Data) ->
+    badreturn;
+idle({call,From}, {delayed_answer,T}, Data) ->
+    receive
+    after T ->
+	    gen_statem:reply({reply,From,delayed}),
+	    throw({keep_state,Data})
+    end;
+idle({call,From}, {timeout,Time}, _Data) ->
+    {next_state,timeout,{From,Time},
+     {timeout,Time,idle}};
+idle(cast, next_event, _Data) ->
+    {next_state,next_events,[a,b,c],
+     [{next_event,internal,a},
+      {next_event,internal,b},
+      {next_event,internal,c}]};
+idle(Type, Content, Data) ->
+    case handle_common_events(Type, Content, idle, Data) of
+	undefined ->
+	    case Type of
+		{call,From} ->
+		    throw({keep_state,Data,[{reply,From,'eh?'}]});
+		_ ->
+		    throw(
+		      {stop,{unexpected,idle,Type,Content}})
+	    end;
+	Result ->
+	    Result
+    end.
+
+timeout(timeout, idle, {From,Time}) ->
+    TRef = erlang:start_timer(Time, self(), ok),
+    {keep_state,{From,TRef},0}; % Immediate timeout 0
+timeout(timeout, 0, {From,TRef}) ->
+    {next_state,timeout2,{From,TRef},
+     [{timeout,1,should_be_cancelled},
+      postpone]}; % Should cancel state timeout
+timeout(_, _, _) ->
+    keep_state_and_data.
+
+timeout2(timeout, 0, _) ->
+    keep_state_and_data;
+timeout2(timeout, Reason, _) ->
+    {stop,Reason};
+timeout2(info, {timeout,TRef,Result}, {From,TRef}) ->
+    gen_statem:reply([{reply,From,Result}]),
+    {next_state,idle,state};
+timeout2(_, _, _) ->
+    {keep_state_and_data,[]}.
+
+wfor_conf({call,From}, confirm, Data) ->
+    {next_state,connected,Data,
+     {reply,From,yes}};
+wfor_conf(cast, {ping,_,_}, _) ->
+    {keep_state_and_data,[postpone]};
+wfor_conf(cast, confirm, Data) ->
+    {next_state,connected,Data};
+wfor_conf(Type, Content, Data) ->
+    case handle_common_events(Type, Content, wfor_conf, Data) of
+	undefined ->
+	    case Type of
+		{call,From} ->
+		    {next_state,idle,Data,
+		     [{reply,From,'eh?'}]};
+		_ ->
+		    throw(keep_state_and_data)
+	    end;
+	Result ->
+	    Result
+    end.
+
+connected({call,From}, {msg,Ref}, Data) ->
+    {keep_state,Data,
+     {reply,From,{ack,Ref}}};
+connected(cast, {msg,From,Ref}, Data) ->
+    From ! {ack,Ref},
+    {keep_state,Data};
+connected({call,From}, disconnect, Data) ->
+    {next_state,idle,Data,
+     [{reply,From,yes}]};
+connected(cast, disconnect, Data) ->
+    {next_state,idle,Data};
+connected(cast, {ping,Pid,Tag}, Data) ->
+    Pid ! {pong,Tag},
+    {keep_state,Data};
+connected(Type, Content, Data) ->
+    case handle_common_events(Type, Content, connected, Data) of
+	undefined ->
+	    case Type of
+		{call,From} ->
+		    {keep_state,Data,
+		     [{reply,From,'eh?'}]};
+		_ ->
+		    {keep_state,Data}
+	    end;
+	Result ->
+	    Result
+    end.
+
+state0({call,From}, stop, Data) ->
+    {stop_and_reply,normal,[{reply,From,stopped}],Data};
+state0(Type, Content, Data) ->
+    case handle_common_events(Type, Content, state0, Data) of
+	undefined ->
+	    {keep_state,Data};
+	Result ->
+	    Result
+    end.
+
+hiber_idle({call,From}, 'alive?', Data) ->
+    {keep_state,Data,
+     [{reply,From,'alive!'}]};
+hiber_idle({call,From}, hibernate_sync, Data) ->
+    {next_state,hiber_wakeup,Data,
+     [{reply,From,hibernating},
+      hibernate]};
+hiber_idle(info, hibernate_later, _) ->
+    Tref = erlang:start_timer(1000, self(), hibernate),
+    {keep_state,Tref};
+hiber_idle(info, hibernate_now, Data) ->
+    {keep_state,Data,
+     [hibernate]};
+hiber_idle(info, {timeout,Tref,hibernate}, Tref) ->
+    {keep_state,[],
+     [hibernate]};
+hiber_idle(cast, hibernate_async, Data) ->
+    {next_state,hiber_wakeup,Data,
+     [hibernate]};
+hiber_idle(Type, Content, Data) ->
+    case handle_common_events(Type, Content, hiber_idle, Data) of
+	undefined ->
+	    {keep_state,Data};
+	Result ->
+	    Result
+    end.
+
+hiber_wakeup({call,From}, wakeup_sync, Data) ->
+    {next_state,hiber_idle,Data,
+     [{reply,From,good_morning}]};
+hiber_wakeup({call,From}, snooze_sync, Data) ->
+    {keep_state,Data,
+     [{reply,From,please_just_five_more},
+      hibernate]};
+hiber_wakeup(cast, wakeup_async, Data) ->
+    {next_state,hiber_idle,Data};
+hiber_wakeup(cast, snooze_async, Data) ->
+    {keep_state,Data,
+     [hibernate]};
+hiber_wakeup(Type, Content, Data) ->
+    case handle_common_events(Type, Content, hiber_wakeup, Data) of
+	undefined ->
+	    {keep_state,Data};
+	Result ->
+	    Result
+    end.
+
+next_events(internal, Msg, [Msg|Msgs]) ->
+    {keep_state,Msgs};
+next_events(Type, Content, Data) ->
+    case handle_common_events(Type, Content, next_events, Data) of
+	undefined ->
+	    {keep_state,Data};
+	Result ->
+	    Result
+    end.
+
+
+handle_common_events({call,From}, get, State, Data) ->
+    {keep_state,Data,
+     [{reply,From,{state,State,Data}}]};
+handle_common_events(cast, {get,Pid}, State, Data) ->
+    Pid ! {state,State,Data},
+    {keep_state,Data};
+handle_common_events({call,From}, stop, _, Data) ->
+    {stop_and_reply,normal,[{reply,From,stopped}],Data};
+handle_common_events(cast, stop, _, _) ->
+    stop;
+handle_common_events({call,From}, {stop,Reason}, _, Data) ->
+    {stop_and_reply,Reason,{reply,From,stopped},Data};
+handle_common_events(cast, {stop,Reason}, _, _) ->
+    {stop,Reason};
+handle_common_events({call,From}, 'alive?', _, Data) ->
+    {keep_state,Data,
+     [{reply,From,yes}]};
+handle_common_events(cast, {'alive?',Pid}, _, Data) ->
+    Pid ! yes,
+    {keep_state,Data};
+handle_common_events(_, _, _, _) ->
+    undefined.
+
+%% Wrapper state machine that uses a map state machine spec
+handle_event(
+  Type, Event, State, [Data|Machine])
+  when is_map(Machine) ->
+    #{State := HandleEvent} = Machine,
+    case
+	try HandleEvent(Type, Event, Data) of
+	    Result ->
+		Result
+	catch
+	    Result ->
+		Result
+	end of
+	{stop,Reason,NewData} ->
+	    {stop,Reason,[NewData|Machine]};
+	{next_state,NewState,NewData} ->
+	    {next_state,NewState,[NewData|Machine]};
+	{next_state,NewState,NewData,Ops} ->
+	    {next_state,NewState,[NewData|Machine],Ops};
+	{keep_state,NewData} ->
+	    {keep_state,[NewData|Machine]};
+	{keep_state,NewData,Ops} ->
+	    {keep_state,[NewData|Machine],Ops};
+	Other ->
+	    Other
+    end;
+%%
+%% Dispatcher to test callback_mode handle_event_function
+%%
+%% Wrap the state in a 1 element list just to test non-atom
+%% states.  Note that the state from init/1 is not wrapped
+%% so both atom and non-atom states are tested.
+handle_event(Type, Event, State, Data) ->
+    StateName = unwrap_state(State),
+    try ?MODULE:StateName(Type, Event, Data) of
+	Result ->
+	    wrap_result(Result)
+    catch
+	throw:Result ->
+	    erlang:raise(
+	      throw, wrap_result(Result), erlang:get_stacktrace())
+    end.
+
+unwrap_state([State]) ->
+    State;
+unwrap_state(State) ->
+    State.
+
+wrap_result(Result) ->
+    case Result of
+	{next_state,NewState,NewData} ->
+	    {next_state,[NewState],NewData};
+	{next_state,NewState,NewData,StateOps} ->
+	    {next_state,[NewState],NewData,StateOps};
+	Other ->
+	    Other
+    end.
+
+
+
+code_change(OldVsn, State, Data, CallbackMode) ->
+    {CallbackMode,State,{OldVsn,Data,CallbackMode}}.
+
+format_status(terminate, [_Pdict,State,Data]) ->
+    {formatted,State,Data};
+format_status(normal, [_Pdict,_State,_Data]) ->
+    [format_status_called].
+
+flush() ->
+    receive
+	Msg ->
+	    [Msg|flush()]
+    after 500 ->
+	    []
+    end.
diff --git a/lib/tools/doc/src/erlang_mode.xml b/lib/tools/doc/src/erlang_mode.xml
index 0ab272fb2f..00cf5196b4 100644
--- a/lib/tools/doc/src/erlang_mode.xml
+++ b/lib/tools/doc/src/erlang_mode.xml
@@ -252,6 +252,7 @@
        behavior</item>
       <item>gen_event - skeleton for the OTP gen_event behavior</item>
       <item>gen_fsm - skeleton for the OTP gen_fsm behavior</item>
+      <item>gen_statem - skeleton for the OTP gen_statem behavior</item>
       <item>Library module - skeleton for a module that does not
        implement a process.</item>
       <item>Corba callback - skeleton for a Corba callback module.</item>
diff --git a/lib/tools/emacs/erlang-skels.el b/lib/tools/emacs/erlang-skels.el
index 3bb1dda6db..ce26c83295 100644
--- a/lib/tools/emacs/erlang-skels.el
+++ b/lib/tools/emacs/erlang-skels.el
@@ -56,6 +56,8 @@
      erlang-skel-gen-event erlang-skel-header)
     ("gen_fsm" "gen-fsm"
      erlang-skel-gen-fsm erlang-skel-header)
+    ("gen_statem" "gen-statem"
+     erlang-skel-gen-statem erlang-skel-header)
     ("wx_object" "wx-object"
      erlang-skel-wx-object erlang-skel-header)
     ("Library module" "gen-lib"
@@ -858,6 +860,122 @@ Please see the function `tempo-define-template'.")
   "*The template of a gen_fsm.
 Please see the function `tempo-define-template'.")
 
+(defvar erlang-skel-gen-statem
+  '((erlang-skel-include erlang-skel-large-header)
+    "-behaviour(gen_statem)." n n
+
+    "%% API" n
+    "-export([start_link/0])." n
+    n
+    "%% gen_statem callbacks" n
+    "-export([init/1, terminate/3, code_change/4])." n
+    "-export([state_name/3])." n
+    "-export([handle_event/4])." n
+    n
+    "-define(SERVER, ?MODULE)." n
+    n
+    "-record(data, {})." n
+    n
+    (erlang-skel-double-separator-start 3)
+    "%%% API" n
+    (erlang-skel-double-separator-end 3) n
+    (erlang-skel-separator-start 2)
+    "%% @doc" n
+    "%% Creates a gen_statem process which calls Module:init/1 to" n
+    "%% initialize. To ensure a synchronized start-up procedure, this" n
+    "%% function does not return until Module:init/1 has returned." n
+    "%%" n
+    (erlang-skel-separator-end 2)
+    "-spec start_link() ->" n>
+    "{ok, Pid :: pid()} |" n>
+    "ignore |" n>
+    "{error, Error :: term()}." n
+    "start_link() ->" n>
+    "gen_statem:start_link({local, ?SERVER}, ?MODULE, [], [])." n
+    n
+    (erlang-skel-double-separator-start 3)
+    "%%% gen_statem callbacks" n
+    (erlang-skel-double-separator-end 3) n
+    (erlang-skel-separator-start 2)
+    "%% @private" n
+    "%% @doc" n
+    "%% Whenever a gen_statem is started using gen_statem:start/[3,4] or" n
+    "%% gen_statem:start_link/[3,4], this function is called by the new" n
+    "%% process to initialize." n
+    (erlang-skel-separator-end 2)
+    "-spec init(Args :: term()) -> " n>
+    "{gen_statem:callback_mode()," n>
+    "State :: term(), Data :: term()} |" n>
+    "{gen_statem:callback_mode()," n>
+    "State :: term(), Data :: term()," n>
+    "[gen_statem:action()] | gen_statem:action()} |" n>
+    "ignore |" n>
+    "{stop, Reason :: term()}." n
+    "init([]) ->" n>
+    "{state_functions, state_name, #data{}}." n
+    n
+    (erlang-skel-separator-start 2)
+    "%% @private" n
+    "%% @doc" n
+    "%% If the gen_statem runs with CallbackMode =:= state_functions" n
+    "%% there should be one instance of this function for each possible" n
+    "%% state name. Whenever a gen_statem receives an event," n
+    "%% the instance of this function with the same name" n
+    "%% as the current state name StateName is called to" n
+    "%% handle the event." n
+    (erlang-skel-separator-end 2)
+    "-spec state_name(" n>
+    "gen_statem:event_type(), Msg :: term()," n>
+    "Data :: term()) ->" n>
+    "gen_statem:state_function_result(). " n
+    "state_name({call,Caller}, _Msg, Data) ->" n>
+    "{next_state, state_name, Data, [{reply,Caller,ok}]}." n
+    n
+    (erlang-skel-separator-start 2)
+    "%% @private" n
+    "%% @doc" n
+    "%% If the gen_statem runs with CallbackMode =:= handle_event_function" n
+    "%% this function is called for every event a gen_statem receives." n
+    (erlang-skel-separator-end 2)
+    "-spec handle_event(" n>
+    "gen_statem:event_type(), Msg :: term()," n>
+    "State :: term(), Data :: term()) ->" n>
+    "gen_statem:handle_event_result(). " n
+    "handle_event({call,From}, _Msg, State, Data) ->" n>
+    "{next_state, State, Data, [{reply,From,ok}]}." n
+    n
+    (erlang-skel-separator-start 2)
+    "%% @private" n
+    "%% @doc" n
+    "%% This function is called by a gen_statem when it is about to" n
+    "%% terminate. It should be the opposite of Module:init/1 and do any" n
+    "%% necessary cleaning up. When it returns, the gen_statem terminates with" n
+    "%% Reason. The return value is ignored." n
+    (erlang-skel-separator-end 2)
+    "-spec terminate(Reason :: term(), State :: term(), Data :: term()) ->" n>
+    "any()." n
+    "terminate(_Reason, _State, _Data) ->" n>
+    "void." n
+    n
+    (erlang-skel-separator-start 2)
+    "%% @private" n
+    "%% @doc" n
+    "%% Convert process state when code is changed" n
+    (erlang-skel-separator-end 2)
+    "-spec code_change(" n>
+    "OldVsn :: term() | {down,term()}," n>
+    "State :: term(), Data :: term(), Extra :: term()) ->" n>
+    "{ok, NewState :: term(), NewData :: term()}." n
+    "code_change(_OldVsn, State, Data, _Extra) ->" n>
+    "{ok, State, Data}." n
+    n
+    (erlang-skel-double-separator-start 3)
+    "%%% Internal functions" n
+    (erlang-skel-double-separator-end 3)
+    )
+  "*The template of a gen_statem.
+Please see the function `tempo-define-template'.")
+
 (defvar erlang-skel-wx-object
   '((erlang-skel-include erlang-skel-large-header)
     "-behaviour(wx_object)." n n
diff --git a/system/doc/definitions/term.defs b/system/doc/definitions/term.defs
index 6091a46a20..921175a7f0 100644
--- a/system/doc/definitions/term.defs
+++ b/system/doc/definitions/term.defs
@@ -76,6 +76,7 @@ the module Erlang in the application kernel","kenneth"},
 {"gen_event","gen_event","A behaviour used for programming event handling mechanisms, such as alarm handlers, error loggers, and plug-and-play handlers.","mbj"},
 {"gen_fsm","gen_fsm","A behaviour used for programming finite state machines.","mbj"},
 {"gen_server","gen_server","A behaviour used for programming client-server processes.","mbj"},
+{"gen_statem","gen_statem","A behaviour used for programming generic state machines.","raimo"},
 {"gterm","Global Glossary Database","A glossary database used to list common acronymns and defintions etc.","jocke"},
 {"xref","xref","A cross reference tool that can be used for finding dependencies between functions, modules, applications and releases. Part of the Tools application.","gunilla"},
 {"GSlong","Graphics System","A library module which provides a graphics interface for Erlang.","mbj"},
diff --git a/system/doc/design_principles/Makefile b/system/doc/design_principles/Makefile
index 653072bc65..937b3e28c8 100644
--- a/system/doc/design_principles/Makefile
+++ b/system/doc/design_principles/Makefile
@@ -58,6 +58,12 @@ GIF_FILES = \
 	sup5.gif \
 	sup6.gif
 
+PNG_FILES = \
+	code_lock.png \
+	code_lock_2.png
+
+IMAGE_FILES = $(GIF_FILES) $(PNG_FILES)
+
 XML_FILES = \
 	$(BOOK_FILES) $(XML_CHAPTER_FILES) \
 	$(XML_PART_FILES) 
@@ -85,13 +91,16 @@ _create_dirs := $(shell mkdir -p $(HTMLDIR))
 $(HTMLDIR)/%.gif: %.gif
 	$(INSTALL_DATA) $< $@
 
+$(HTMLDIR)/%.png: %.png
+	$(INSTALL_DATA) $< $@
+
 docs: html
 
 local_docs: PDFDIR=../../pdf
 
-html: $(HTML_UG_FILE) gifs
+html: $(HTML_UG_FILE) images
 
-gifs: $(GIF_FILES:%=$(HTMLDIR)/%)
+images: $(IMAGE_FILES:%=$(HTMLDIR)/%)
 
 debug opt: 
 
@@ -109,7 +118,7 @@ release_docs_spec: docs
 #	$(INSTALL_DIR) "$(RELEASE_PATH)/pdf"
 #	$(INSTALL_DATA) $(TOP_PDF_FILE) "$(RELEASE_PATH)/pdf"
 	$(INSTALL_DIR) $(RELSYSDIR)
-	$(INSTALL_DATA) $(GIF_FILES) $(HTMLDIR)/*.html \
+	$(INSTALL_DATA) $(IMAGE_FILES) $(HTMLDIR)/*.html \
 		$(RELSYSDIR)
 
 
diff --git a/system/doc/design_principles/appup_cookbook.xml b/system/doc/design_principles/appup_cookbook.xml
index 417b482fba..4f23c42c59 100644
--- a/system/doc/design_principles/appup_cookbook.xml
+++ b/system/doc/design_principles/appup_cookbook.xml
@@ -50,7 +50,8 @@
     <p>In a system implemented according to the OTP design principles,
       all processes, except system processes and special processes,
       reside in one of the behaviours <c>supervisor</c>,
-      <c>gen_server</c>, <c>gen_fsm</c>, or <c>gen_event</c>. These
+      <c>gen_server</c>, <c>gen_fsm</c>,
+      <c>gen_statem</c> or <c>gen_event</c>. These
       belong to the STDLIB application and upgrading/downgrading
       normally requires an emulator restart.</p>
     <p>OTP thus provides no support for changing residence modules except
diff --git a/system/doc/design_principles/code_lock.dia b/system/doc/design_principles/code_lock.dia
new file mode 100644
index 0000000000..bed6d8ee86
--- /dev/null
+++ b/system/doc/design_principles/code_lock.dia
diff --git a/system/doc/design_principles/code_lock.png b/system/doc/design_principles/code_lock.png
new file mode 100644
index 0000000000..e40f0320aa
--- /dev/null
+++ b/system/doc/design_principles/code_lock.png
diff --git a/system/doc/design_principles/code_lock_2.dia b/system/doc/design_principles/code_lock_2.dia
new file mode 100644
index 0000000000..4e82a9e1d6
--- /dev/null
+++ b/system/doc/design_principles/code_lock_2.dia
diff --git a/system/doc/design_principles/code_lock_2.png b/system/doc/design_principles/code_lock_2.png
new file mode 100644
index 0000000000..138fbdef6c
--- /dev/null
+++ b/system/doc/design_principles/code_lock_2.png
diff --git a/system/doc/design_principles/des_princ.xml b/system/doc/design_principles/des_princ.xml
index 0cf9f28bdc..8ab8661c2d 100644
--- a/system/doc/design_principles/des_princ.xml
+++ b/system/doc/design_principles/des_princ.xml
@@ -226,7 +226,9 @@ free(Ch, {Alloc, Free} = Channels) ->
       <item><p><seealso marker="gen_server_concepts">gen_server</seealso></p>
       <p>For implementing the server of a client-server relation</p></item>
       <item><p><seealso marker="fsm">gen_fsm</seealso></p>
-      <p>For implementing finite-state machines</p></item>
+      <p>For implementing finite-state machines (Old)</p></item>
+      <item><p><seealso marker="statem">gen_statem</seealso></p>
+      <p>For implementing state machines (New)</p></item>
       <item><p><seealso marker="events">gen_event</seealso></p>
       <p>For implementing event handling functionality</p></item>
       <item><p><seealso marker="sup_princ">supervisor</seealso></p>
diff --git a/system/doc/design_principles/fsm.xml b/system/doc/design_principles/fsm.xml
index f20d20fb7e..3468f93ae0 100644
--- a/system/doc/design_principles/fsm.xml
+++ b/system/doc/design_principles/fsm.xml
@@ -30,6 +30,16 @@
     <file>fsm.xml</file>
   </header>
   <marker id="gen_fsm behaviour"></marker>
+    <note>
+      <p>
+	There is a new behaviour
+	<seealso marker="gen_statem"><c>gen_statem</c></seealso>
+	that is intended to replace <c>gen_fsm</c> for new code.
+	It has the same features and add some really useful.
+	This module will not be removed for the foreseeable future
+	to keep old state machine implementations running.
+      </p>
+    </note>
   <p>This section is to be read with the <c>gen_fsm(3)</c> manual page
     in STDLIB, where all interface functions and callback
     functions are described in detail.</p>
diff --git a/system/doc/design_principles/part.xml b/system/doc/design_principles/part.xml
index 7862a2d650..6495211e04 100644
--- a/system/doc/design_principles/part.xml
+++ b/system/doc/design_principles/part.xml
@@ -31,6 +31,7 @@
   <xi:include href="des_princ.xml"/>
   <xi:include href="gen_server_concepts.xml"/>
   <xi:include href="fsm.xml"/>
+  <xi:include href="statem.xml"/>
   <xi:include href="events.xml"/>
   <xi:include href="sup_princ.xml"/>
   <xi:include href="spec_proc.xml"/>
diff --git a/system/doc/design_principles/release_handling.xml b/system/doc/design_principles/release_handling.xml
index c38cca248f..4f71ad4437 100644
--- a/system/doc/design_principles/release_handling.xml
+++ b/system/doc/design_principles/release_handling.xml
@@ -249,7 +249,7 @@
       <p>If <c>Modules=dynamic</c>, which is the case for event
         managers, the event manager process informs the release handler
         about the list of currently installed event handlers
-	(<c>gen_fsm</c>), and it is checked if the module name is in
+	(<c>gen_event</c>), and it is checked if the module name is in
         this list instead.</p>
       <p>The release handler suspends, asks for code change, and
         resumes processes by calling the functions
diff --git a/system/doc/design_principles/statem.xml b/system/doc/design_principles/statem.xml
new file mode 100644
index 0000000000..a4b8fb06a0
--- /dev/null
+++ b/system/doc/design_principles/statem.xml
@@ -0,0 +1,1451 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<!DOCTYPE chapter SYSTEM "chapter.dtd">
+
+<chapter>
+  <header>
+    <copyright>
+      <year>2016</year>
+      <holder>Ericsson AB.  All Rights Reserved.</holder>
+    </copyright>
+    <legalnotice>
+      Licensed under the Apache License, Version 2.0 (the "License");
+      you may not use this file except in compliance with the License.
+      You may obtain a copy of the License at
+
+          http://www.apache.org/licenses/LICENSE-2.0
+
+      Unless required by applicable law or agreed to in writing, software
+      distributed under the License is distributed on an "AS IS" BASIS,
+      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+      See the License for the specific language governing permissions and
+      limitations under the License.
+
+    </legalnotice>
+
+    <title>gen_statem Behaviour</title>
+    <prepared></prepared>
+    <docno></docno>
+    <date></date>
+    <rev></rev>
+    <file>statem.xml</file>
+  </header>
+  <marker id="gen_statem behaviour"></marker>
+  <p>
+    This section is to be read with the
+    <seealso marker="stdlib:gen_statem"><c>gen_statem(3)</c></seealso>
+    manual page in STDLIB, where all interface functions and callback
+    functions are described in detail.
+  </p>
+  <p>
+    This is a new behaviour in OTP-19.0.
+    It has been thoroughly reviewed, is stable enough
+    to be used by at least two heavy OTP applications, and is here to stay.
+    But depending on user feedback, we do not expect
+    but might find it necessary to make minor
+    not backwards compatible changes into OTP-20.0,
+    so its state can be designated as "not quite experimental"...
+  </p>
+
+<!-- =================================================================== -->
+
+  <section>
+    <title>Event Driven State Machines</title>
+    <p>
+      Established Automata theory does not deal much with
+      how a state transition is triggered,
+      but in general assumes that the output is a function
+      of the input (and the state) and that they are
+      some kind of values.
+    </p>
+    <p>
+      For an Event Driven State Machine the input is an event
+      that triggers a state transition and the output
+      is actions executed during the state transition.
+      It can analogously to the mathematical model of a
+      Finite State Machine be described as
+      a set of relations of the form:
+    </p>
+    <pre>
+State(S) x Event(E) -> Actions(A), State(S')</pre>
+    <p>These relations are interpreted as meaning:</p>
+    <p>
+      If we are in state <c>S</c> and event <c>E</c> occurs, we
+      are to perform actions <c>A</c> and make a transition to
+      state <c>S'</c>.
+    </p>
+    <p>
+      Note that <c>S'</c> may be equal to <c>S</c>.
+    </p>
+    <p>
+      Since <c>A</c> and <c>S'</c> depend only on
+      <c>S</c> and <c>E</c> the kind of state machine described
+      here is a Mealy Machine.
+      (See for example the corresponding Wikipedia article)
+    </p>
+    <p>
+      Like most <c>gen_</c> behaviours, <c>gen_statem</c> keeps
+      a server <c>Data</c> besides the state.  This and the fact that
+      there is no restriction on the number of states
+      (assuming enough virtual machine memory)
+      or on the number of distinct input events actually makes
+      a state machine implemented with this behaviour Turing complete.
+      But it feels mostly like an Event Driven Mealy Machine.
+    </p>
+  </section>
+
+<!-- =================================================================== -->
+
+  <section>
+    <marker id="callback_modes" />
+    <title>Callback Modes</title>
+    <p>
+      The <c>gen_statem</c> behaviour supports two different callback modes.
+      In the mode
+      <seealso marker="stdlib:gen_statem#type-callback_mode">
+	<c>state_functions</c>,
+      </seealso>
+      the state transition rules are written as a number of Erlang
+      functions, which conform to the following convention:
+    </p>
+    <pre>
+StateName(EventType, EventContent, Data) ->
+    .. code for actions here ...
+    {next_state, NewStateName, NewData}.</pre>
+    <p>
+      In the mode
+      <seealso marker="stdlib:gen_statem#type-callback_mode">
+	<c>handle_event_function</c>
+      </seealso>
+      there is only one
+      Erlang function that implements all state transition rules:
+    </p>
+    <pre>
+handle_event(EventType, EventContent, State, Data) ->
+    .. code for actions here ...
+    {next_state, State', Data'}</pre>
+    <p>
+      Both these modes allow other return tuples
+      that you can find in the
+      <seealso marker="stdlib:gen_statem#Module:StateName/3">
+	reference manual.
+      </seealso>
+      These other return tuples can for example stop the machine,
+      execute state transition actions on the machine engine itself
+      and send replies.
+    </p>
+
+    <section>
+      <title>Choosing the Callback Mode</title>
+      <p>
+	The two
+	<seealso marker="#callback_modes">callback modes</seealso>
+	gives different possibilities
+	and restrictions, but one goal remains:
+	you want to handle all possible combinations of
+	events and states.
+      </p>
+      <p>
+	You can for example do this by focusing on one state at the time
+	and for every state ensure that all events are handled,
+	or the other way around focus on one event at the time
+	and ensure that it is handled in every state,
+	or mix these strategies.
+      </p>
+      <p>
+	With <c>state_functions</c> you are restricted to use
+	atom only states, and the <c>gen_statem</c> engine dispatches
+	on state name for you.  This encourages the callback module
+	to gather the implementation of all event actions particular
+	to one state in the same place in the code
+	hence to focus on one state at the time.
+      </p>
+      <p>
+	This mode fits well when you have a regular state diagram
+	like the ones in this chapter that describes all events and actions
+	belonging to a state visually around that state,
+	and each state has its unique name.
+      </p>
+      <p>
+	With <c>handle_event_function</c> you are free to mix strategies
+	as you like because all events and states
+	are handled in the the same callback function.
+      </p>
+      <p>
+	This mode works equally well when you want to focus on
+	one event at the time or when you want to focus on
+	one state at the time, but the <c>handle_event/4</c> function
+	quickly grows too large to handle without introducing dispatching.
+      </p>
+      <p>
+	The mode enables the use of non-atom states for example
+	complex states or even hiearchical states.
+	If, for example, a state diagram is largely alike
+	for the client and for the server side of a protocol;
+	then you can have a state <c>{StateName,server}</c> or
+	<c>{StateName,client}</c> and since you do the dispatching
+	yourself you make <c>StateName</c> decide where in the code
+	to handle most events in the state.
+	The second element of the tuple	is then used to select
+	whether to handle special client side or server side events.
+      </p>
+    </section>
+  </section>
+
+<!-- =================================================================== -->
+
+  <section>
+    <title>Example</title>
+    <p>
+      This is an example starting off as equivalent to the the example in the
+      <seealso marker="fsm"><c>gen_fsm</c> behaviour</seealso>
+      description.  In later chapters additions and tweaks are made
+      using features in <c>gen_statem</c> that <c>gen_fsm</c> does not have.
+      At the end of this section you can find the example again
+      with all the added features.
+    </p>
+    <p>
+      A door with a code lock can be viewed as a state machine.
+      Initially, the door is locked.  Anytime someone presses a button,
+      this generates an event.
+      Depending on what buttons have been pressed before,
+      the sequence so far can be correct, incomplete, or wrong.
+    </p>
+    <p>
+      If it is correct, the door is unlocked for 10 seconds (10000 ms).
+      If it is incomplete, we wait for another button to be pressed.  If
+      it is is wrong, we start all over,
+      waiting for a new button sequence.
+    </p>
+    <image file="../design_principles/code_lock.png">
+      <icaption>Code lock state diagram</icaption>
+    </image>
+    <p>
+      We can implement such a code lock state machine using
+      <c>gen_statem</c> with the following callback module:
+    </p>
+    <marker id="ex"></marker>
+    <code type="erl"><![CDATA[
+-module(code_lock).
+-behaviour(gen_statem).
+-define(NAME, code_lock).
+-define(CALLBACK_MODE, state_functions).
+
+-export([start_link/1]).
+-export([button/1]).
+-export([init/1,terminate/3,code_change/4]).
+-export([locked/3,open/3]).
+
+start_link(Code) ->
+    gen_statem:start_link({local,?NAME}, ?MODULE, Code, []).
+
+button(Digit) ->
+    gen_statem:cast(?NAME, {button,Digit}).
+
+
+init(Code) ->
+    do_lock(),
+    Data = #{code => Code, remaining => Code},
+    {?CALLBACK_MODE,locked,Data}.
+
+locked(
+  cast, {button,Digit},
+  #{code := Code, remaining := Remaining} = Data) ->
+    case Remaining of
+        [Digit] ->
+	    do_unlock(),
+            {next_state,open,Data#{remaining := Code},10000};
+        [Digit|Rest] -> % Incomplete
+            {next_state,locked,Data#{remaining := Rest}};
+        _Wrong ->
+            {next_state,locked,Data#{remaining := Code}}
+    end.
+
+open(timeout, _,  Data) ->
+    do_lock(),
+    {next_state,locked,Data};
+open(cast, {button,_}, Data) ->
+    do_lock(),
+    {next_state,locked,Data}.
+
+do_lock() ->
+    io:format("Lock~n", []).
+do_unlock() ->
+    io:format("Unlock~n", []).
+
+terminate(_Reason, State, _Data) ->
+    State =/= locked andalso do_lock(),
+    ok.
+code_change(_Vsn, State, Data, _Extra) ->
+    {?CALLBACK_MODE,State,Data}.
+    ]]></code>
+    <p>The code is explained in the next sections.</p>
+  </section>
+
+<!-- =================================================================== -->
+
+  <section>
+    <title>Starting gen_statem</title>
+    <p>
+      In the example in the previous section, the <c>gen_statem</c> is
+      started by calling <c>code_lock:start_link(Code)</c>:
+    </p>
+    <code type="erl"><![CDATA[
+start_link(Code) ->
+    gen_statem:start_link({local,?NAME}, ?MODULE, Code, []).
+    ]]></code>
+    <p>
+      <c>start_link</c> calls the function
+      <seealso marker="stdlib:gen_statem#start_link/4">
+	<c>gen_statem:start_link/4</c>
+      </seealso>
+      which spawns and links to a new process; a <c>gen_statem</c>.
+    </p>
+    <list type="bulleted">
+      <item>
+        <p>
+	  The first argument, <c>{local,?NAME}</c>, specifies
+          the name.  In this case, the <c>gen_statem</c> is locally
+	  registered as <c>code_lock</c> through the macro <c>?NAME</c>.
+	</p>
+        <p>
+	  If the name is omitted, the <c>gen_statem</c> is not registered.
+          Instead its pid must be used.  The name can also be given
+	  as <c>{global,Name}</c>, in which case the <c>gen_statem</c> is
+	  registered using
+	  <seealso marker="kernel:global#register_name/2">
+	    <c>global:register_name/2</c>.
+	  </seealso>
+	</p>
+      </item>
+      <item>
+        <p>
+	  The second argument, <c>?MODULE</c>, is the name of
+          the callback module, that is; the module where the callback
+          functions are located, which is this module.
+	</p>
+        <p>
+	  The interface functions (<c>start_link/1</c> and <c>button/1</c>)
+	  are located in the same module as the callback functions
+	  (<c>init/1</c>, <c>locked/3</c>, and <c>open/3</c>).
+	  It is normally good programming practice to have the client
+	  side and the server side code contained in one module.
+	</p>
+      </item>
+      <item>
+        <p>
+	  The third argument, <c>Code</c>, is a list of digits that
+	  is the correct unlock code which is passsed
+	  to the callback function <c>init/1</c>.
+	</p>
+      </item>
+      <item>
+        <p>
+	  The fourth argument, <c>[]</c>, is a list of options.  See the
+	  <seealso marker="stdlib:gen_statem#start_link/3">
+	    <c>gen_statem:start_link/3</c>
+	  </seealso>
+	  manual page for available options.
+	</p>
+      </item>
+    </list>
+    <p>
+      If name registration succeeds, the new <c>gen_statem</c> process
+      calls the callback function <c>code_lock:init(Code)</c>.
+      This function is expected to return <c>{CallbackMode,State,Data}</c>,
+      where
+      <seealso marker="#callback_modes">
+	<c>CallbackMode</c>
+      </seealso>
+      selects callback module state function mode, in this case
+      <seealso marker="stdlib:gen_statem#type-callback_mode">
+	<c>state_functions</c>
+      </seealso>
+      through the macro <c>?CALLBACK_MODE</c> that is; each state
+      has got its own handler function.
+      <c>State</c> is the initial state of the <c>gen_statem</c>,
+      in this case <c>locked</c>; assuming the door is locked to begin with.
+      <c>Data</c> is the internal server data of the <c>gen_statem</c>.
+      Here the server data is a <seealso marker="stdlib:maps">map</seealso>
+      with the key <c>code</c> that stores
+      the correct button sequence and the key <c>remaining</c>
+      that stores the remaining correct button sequence
+      (the same as the <c>code</c> to begin with).
+    </p>
+    <code type="erl"><![CDATA[
+init(Code) ->
+    do_lock(),
+    Data = #{code => Code, remaining => Code},
+    {?CALLBACK_MODE,locked,Data}.
+    ]]></code>
+    <p>
+      <seealso marker="stdlib:gen_statem#start_link/3">
+	<c>gen_statem:start_link</c>
+      </seealso>
+      is synchronous.  It does not return until the <c>gen_statem</c>
+      has been initialized and is ready to receive events.
+    </p>
+    <p>
+      <seealso marker="stdlib:gen_statem#start_link/3">
+	<c>gen_statem:start_link</c>
+      </seealso>
+      must be used if the <c>gen_statem</c>
+      is part of a supervision tree, that is; started by a supervisor.
+      There is another function;
+      <seealso marker="stdlib:gen_statem#start/3">
+	<c>gen_statem:start</c>
+      </seealso>
+      to start a standalone <c>gen_statem</c>, that is;
+      a <c>gen_statem</c> that is not part of a supervision tree.
+    </p>
+  </section>
+
+<!-- =================================================================== -->
+
+  <section>
+    <title>Events and Handling them</title>
+    <p>The function notifying the code lock about a button event is
+      implemented using
+      <seealso marker="stdlib:gen_statem#cast/2">
+	<c>gen_statem:cast/2</c>:
+      </seealso>
+    </p>
+    <code type="erl"><![CDATA[
+button(Digit) ->
+    gen_statem:cast(?NAME, {button,Digit}).
+    ]]></code>
+    <p>
+      The first argument is the name of the <c>gen_statem</c> and must
+      agree with the name used to start it so therefore we use the
+      same macro <c>?NAME</c> as when starting.
+      <c>{button,Digit}</c> is the actual event content.
+    </p>
+    <p>
+      The event is made into a message and sent to the <c>gen_statem</c>.
+      When the event is received, the <c>gen_statem</c> calls
+      <c>StateName(cast, Event, Data)</c>, which is expected to
+      return a tuple <c>{next_state,NewStateName,NewData}</c>.
+      <c>StateName</c> is the name of the current state and
+      <c>NewStateName</c> is the name of the next state to go to.
+      <c>NewData</c> is a new value for the server data of
+      the <c>gen_statem</c>.
+    </p>
+    <code type="erl"><![CDATA[
+locked(
+  cast, {button,Digit},
+  #{code := Code, remaining := Remaining} = Data) ->
+    case Remaining of
+        [Digit] -> % Complete
+	    do_unlock(),
+            {next_state,open,Data#{remaining := Code},10000};
+        [Digit|Rest] -> % Incomplete
+            {next_state,locked,Data#{remaining := Rest}};
+        [_|_] -> % Wrong
+            {next_state,locked,Data#{remaining := Code}}
+    end.
+
+open(timeout, _, Data) ->
+    do_lock(),
+    {next_state,locked,Data};
+open(cast, {button,_}, Data) ->
+    do_lock(),
+    {next_state,locked,Data}.
+    ]]></code>
+    <p>
+      If the door is locked and a button is pressed, the pressed
+      button is compared with the next correct button and,
+      depending on the result, the door is either unlocked
+      and the <c>gen_statem</c> goes to state <c>open</c>,
+      or the door remains in state <c>locked</c>.
+    </p>
+    <p>
+      If the pressed button is incorrect the server data
+      restarts from the start of the code sequence.
+    </p>
+    <p>
+      In state <c>open</c> any button locks the door since
+      any event cancels the event timer so we will not get
+      a timeout event after a button event.
+    </p>
+  </section>
+
+  <section>
+    <title>Event Time-Outs</title>
+    <p>
+      When a correct code has been given, the door is unlocked and
+      the following tuple is returned from <c>locked/2</c>:
+    </p>
+    <code type="erl"><![CDATA[
+{next_state,open,Data#{remaining := Code},10000};
+    ]]></code>
+    <p>
+      10000 is a time-out value in milliseconds.
+      After this time, that is; 10 seconds, a time-out occurs.
+      Then, <c>StateName(timeout, 10000, Data)</c> is called.
+      The time-out occurs when the door has been in state <c>open</c>
+      for 10 seconds.  After that the door is locked again:
+    </p>
+    <code type="erl"><![CDATA[
+open(timeout, _,  Data) ->
+    do_lock(),
+    {next_state,locked,Data};
+    ]]></code>
+  </section>
+
+<!-- =================================================================== -->
+
+  <section>
+    <title>All State Events</title>
+    <p>
+      Sometimes an event can arrive in any state of the <c>gen_statem</c>.
+      It is convenient to handle these in a common state handler function
+      that all state functions call for events not specific to the state.
+    </p>
+    <p>
+      Let's introduce a <c>code_length/0</c> function that returns
+      the length of the correct code
+      (that should not be sensitive to reveal...).
+      We'll dispatch all events that are not state specific
+      to the common function <c>handle_event/3</c>.
+    </p>
+    <code type="erl"><![CDATA[
+...
+-export([button/1,code_length/0]).
+...
+
+code_length() ->
+    gen_statem:call(?NAME, code_length).
+
+...
+locked(...) -> ... ;
+locked(EventType, EventContent, Data) ->
+    handle_event(EventType, EventContent, Data).
+
+...
+open(...) -> ... ;
+open(EventType, EventContent, Data) ->
+    handle_event(EventType, EventContent, Data).
+
+handle_event({call,From}, code_length, #{code := Code} = Data) ->
+    {keep_state,Data,[{reply,From,length(Code)}]}.
+    ]]></code>
+    <p>
+      This example uses
+      <seealso marker="stdlib:gen_statem#call/2">
+	<c>gen_statem:call/2</c>
+      </seealso>
+      which waits for a reply from the server.
+      The reply is sent with a <c>{reply,From,Reply}</c> tuple
+      in an action list in the <c>{keep_state,...}</c> tuple
+      that retains the current state.
+    </p>
+  </section>
+
+<!-- =================================================================== -->
+
+  <section>
+    <title>One Event Handler</title>
+    <p>
+      If you use the mode <c>handle_event_function</c>
+      all events are handled in <c>handle_event/4</c> and we
+      may (but do not have to) use an event-centered approach
+      where we dispatch on event first and then state:
+    </p>
+    <code type="erl"><![CDATA[
+...
+-define(CALLBACK_MODE, state_functions).
+
+...
+-export([handle_event/4]).
+
+...
+
+handle_event(cast, {button,Digit}, State, #{code := Code} = Data) ->
+    case State of
+	locked ->
+	    case maps:get(remaining, Data) of
+		[Digit] -> % Complete
+		    do_unlock(),
+		    {next_state,open,Data#{remaining := Code},10000};
+		[Digit|Rest] -> % Incomplete
+		    {keep_state,Data#{remaining := Rest}};
+		[_|_] -> % Wrong
+		    {keep_state,Data#{remaining := Code}}
+	    end;
+	open ->
+	    do_lock(),
+	    {next_state,locked,Data}
+    end;
+handle_event(timeout, _, open, Data) ->
+    do_lock(),
+    {next_state,locked,Data}.
+
+...
+    ]]></code>
+  </section>
+
+<!-- =================================================================== -->
+
+  <section>
+    <title>Stopping</title>
+
+    <section>
+      <title>In a Supervision Tree</title>
+      <p>
+	If the <c>gen_statem</c> is part of a supervision tree,
+	no stop function is needed.
+	The <c>gen_statem</c> is automatically terminated by its supervisor.
+	Exactly how this is done is defined by a
+	<seealso marker="sup_princ#shutdown">shutdown strategy</seealso>
+        set in the supervisor.
+      </p>
+      <p>
+	If it is necessary to clean up before termination, the shutdown
+        strategy must be a time-out value and the <c>gen_statem</c> must
+	in the <c>init/1</c> function set itself to trap exit signals
+	by calling
+	<seealso marker="erts:erlang#process_flag/2">
+	  <c>process_flag(trap_exit, true)</c>.
+	</seealso>
+	When ordered to shutdown, the <c>gen_statem</c> then calls
+	the callback function
+	<c>terminate(shutdown, State, Data)</c>:
+      </p>
+      <code type="erl"><![CDATA[
+init(Args) ->
+    process_flag(trap_exit, true),
+    do_lock(),
+    ...
+      ]]></code>
+      <p>
+	In this example we let the <c>terminate/3</c> function
+	lock the door if it is open so we do not accidentally leave the door
+	open when the supervision tree terminates.
+      </p>
+      <code type="erl"><![CDATA[
+terminate(_Reason, State, _Data) ->
+    State =/= locked andalso do_lock(),
+    ok.
+      ]]></code>
+    </section>
+
+    <section>
+      <title>Standalone gen_statem</title>
+      <p>
+	If the <c>gen_statem</c> is not part of a supervision tree,
+	it can be stopped using
+	<seealso marker="stdlib:gen_statem#stop/1">
+	  <c>gen_statem:stop</c>,
+	</seealso>
+	preferably through an API function:
+      </p>
+      <code type="erl"><![CDATA[
+...
+-export([start_link/1,stop/0]).
+
+...
+stop() ->
+    gen_statem:stop(?NAME).
+      ]]></code>
+      <p>
+	This makes the <c>gen_statem</c> call the <c>terminate/3</c>
+	callback function just like for a supervised server
+	and waits for the process to terminate.
+      </p>
+    </section>
+  </section>
+
+<!-- =================================================================== -->
+
+  <section>
+    <title>Actions</title>
+    <p>
+      In the first chapters we mentioned actions as a part of
+      the general state machine model, and these actions
+      are implemented with the code the <c>gen_statem</c>
+      callback module executes in an event handling
+      callback function before returning
+      to the <c>gen_statem</c> engine.
+    </p>
+    <p>
+      There are more specific state transition actions
+      that a callback function can order the <c>gen_statem</c>
+      engine to do after the callback function return.
+      These are ordered by returning a list of
+      <seealso marker="stdlib:gen_statem#type-action">
+	actions
+      </seealso>
+      in the
+      <seealso marker="stdlib:gen_statem#type-state_function_result">
+	return tuple
+      </seealso>
+      from the
+      <seealso marker="stdlib:gen_statem#Module:StateName/3">
+	callback function.
+      </seealso>
+      These state transition actions affect the <c>gen_statem</c>
+      engine itself.  They can:
+    </p>
+    <list type="bulleted">
+      <item>Postpone the current event.</item>
+      <item>Hibernate the <c>gen_statem</c>.</item>
+      <item>Start an event timeout.</item>
+      <item>Reply to a caller.</item>
+      <item>Generate the next event to handle.</item>
+    </list>
+    <p>
+      We have mentioned the event timeout
+      and replying to a caller in the example above.
+      An example of event postponing comes in later in this chapter.
+      See the
+      <seealso marker="stdlib:gen_statem#type-action">
+	reference manual
+      </seealso>
+      for details.  You can for example actually reply to several callers
+      and generate multiple next events to handle.
+    </p>
+  </section>
+
+<!-- =================================================================== -->
+
+  <section>
+    <title>Event Types</title>
+    <p>
+      So far we have mentioned a few
+      <seealso marker="stdlib:gen_statem#type-event_type">
+	event types.
+      </seealso>
+      Events of all types are handled in the same callback function,
+      for a given state, and the function gets
+      <c>EventType</c> and <c>EventContent</c> as arguments.
+    </p>
+    <p>
+      Here is the complete list of event types and where
+      they come from:
+    </p>
+    <taglist>
+      <tag><c>cast</c></tag>
+      <item>
+	Generated by
+	<seealso marker="stdlib:gen_statem#cast/2">
+	  <c>gen_statem:cast</c>.
+	</seealso>
+      </item>
+      <tag><c>{call,From}</c></tag>
+      <item>
+	Generated by
+	<seealso marker="stdlib:gen_statem#call/2">
+	  <c>gen_statem:call</c>
+	</seealso>
+	where <c>From</c> is the reply address to use
+	when replying either through the state transition action
+	<c>{reply,From,Msg}</c> or by calling
+	<seealso marker="stdlib:gen_statem#reply/1">
+	  <c>gen_statem:reply</c>.
+	</seealso>
+      </item>
+      <tag><c>info</c></tag>
+      <item>
+	Generated by any regular process message sent to
+	the <c>gen_statem</c> process.
+      </item>
+      <tag><c>timeout</c></tag>
+      <item>
+	Generated by the state transition action
+	<c>{timeout,Time,EventContent}</c> (or its short form <c>Time</c>)
+	timer timing out.
+      </item>
+      <tag><c>internal</c></tag>
+      <item>
+	Generated by the state transition action
+	<c>{next_event,internal,EventContent}</c>.
+	In fact all event types above can be generated using
+	<c>{next_event,EventType,EventContent}</c>.
+      </item>
+    </taglist>
+  </section>
+
+<!-- =================================================================== -->
+
+  <section>
+    <title>State Timeouts</title>
+    <p>
+      The timeout event generated by the state transition action
+      <c>{timeout,Time,EventContent}</c> is an event timeout,
+      that is; if an event arrives the timer is cancelled.
+      You get either an event or a timeout but not both.
+    </p>
+    <p>
+      Often you want a timer to not be cancelled by any event
+      or you want to start a timer in one state and respond
+      to the timeout in another.  This can be accomplished
+      with a regular erlang timer:
+      <seealso marker="erts:erlang#start_timer/4">
+	<c>erlang:start_timer</c>.
+      </seealso>
+    </p>
+    <p>
+      Looking at the example in this chapter so far; using the
+      <c>gen_statem</c> event timer has the consequence that
+      if a button event is generated while in the <c>open</c> state,
+      the timeout is cancelled and the button event is delivered.
+      Therefore we chose to lock the door if this happended.
+    </p>
+    <p>
+      Suppose we do not want a button to lock the door,
+      instead we want to ignore button events in the <c>open</c> state.
+      Then we start a timer when entering the <c>open</c> state
+      and wait for it to expire while ignoring button events:
+    </p>
+    <code type="erl"><![CDATA[
+...
+locked(
+  cast, {button,Digit},
+  #{code := Code, remaining := Remaining} = Data) ->
+    case Remaining of
+        [Digit] ->
+	    do_unlock(),
+	    Tref = erlang:start_timer(10000, self(), lock),
+            {next_state,open,Data#{remaining := Code, timer := Tref}};
+...
+
+open(info, {timeout,Tref,lock}, #{timer := Tref} = Data) ->
+    do_lock(),
+    {next_state,locked,Data};
+open(cast, {button,_}, Data) ->
+    {keep_state,Data};
+...
+    ]]></code>
+    <p>
+      If you need to cancel a timer due to some other event you can use
+      <seealso marker="erts:erlang#cancel_timer/2">
+	<c>erlang:cancel_timer(Tref)</c>.
+      </seealso>
+      Note that a timeout message can not arrive after this,
+      unless you have postponed it (see the next section) before,
+      so make sure you do not accidentally postpone such messages.
+    </p>
+    <p>
+      Another way to cancel a timer is to not cancel it,
+      but instead to ignore it if it arrives in a state
+      where it is known to be late.
+    </p>
+  </section>
+
+<!-- =================================================================== -->
+
+  <section>
+    <title>Postponing Events</title>
+    <p>
+      If you want to ignore a particular event in the current state
+      and handle it in a future state, you can postpone the event.
+      A postponed event is retried after the state has
+      changed i.e <c>OldState =/= NewState</c>.
+    </p>
+    <p>
+      Postponing is ordered by the state transition
+      <seealso marker="stdlib:gen_statem#type-action">
+	action
+      </seealso>
+      <c>postpone</c>.
+    </p>
+    <p>
+      In this example, instead of ignoring button events
+      while in the <c>open</c> state we can postpone them
+      and they will be queued and later handled in the <c>locked</c> state:
+    </p>
+    <code type="erl"><![CDATA[
+...
+open(cast, {button,_}, Data) ->
+    {keep_state,Data,[postpone]};
+...
+    ]]></code>
+    <p>
+      The fact that a postponed event is only retried after a state change
+      translates into a requirement on the event and state space:
+      if you have a choice between storing a state data item
+      in the <c>State</c> or in the <c>Data</c>;
+      should a change in the item value affect which events that
+      are handled, then this item ought to be part of the state.
+    </p>
+    <p>
+      What you want to avoid is that you maybe much later decide
+      to postpone an event in one state and by misfortune it is never retried
+      because the code only changes the <c>Data</c> but not the <c>State</c>.
+    </p>
+
+    <section>
+      <title>Fuzzy State Diagrams</title>
+      <p>
+	It is not uncommon that a state diagram does not specify
+	how to handle events that are not illustrated
+	in a particular state in the diagram.
+	Hopefully this is described in an associated text
+	or from the context.
+      </p>
+      <p>
+	Possible actions may be; ignore as in drop the event
+	(maybe log it) or deal with the event in some other state
+	as in postpone it.
+      </p>
+    </section>
+
+    <section>
+      <title>Selective Receive</title>
+      <p>
+	Erlang's selective receive statement is often used to
+	describe simple state machine examples in straightforward
+	Erlang code.  Here is a possible implementation of
+	the first example:
+      </p>
+    <code type="erl"><![CDATA[
+-module(code_lock).
+-define(NAME, code_lock_1).
+-export([start_link/1,button/1]).
+
+start_link(Code) ->
+    spawn(
+      fun () ->
+	      true = register(?NAME, self()),
+	      do_lock(),
+	      locked(Code, Code)
+      end).
+
+button(Digit) ->
+    ?NAME ! {button,Digit}.
+
+locked(Code, [Digit|Remaining]) ->
+    receive
+	{button,Digit} when Remaining =:= [] ->
+	    do_unlock(),
+	    open(Code);
+	{button,Digit} ->
+	    locked(Code, Remaining);
+	{button,_} ->
+	    locked(Code, Code)
+    end.
+
+open(Code) ->
+    receive
+    after 10000 ->
+	    do_lock(),
+	    locked(Code, Code)
+    end.
+
+do_lock() ->
+    io:format("Locked~n", []).
+do_unlock() ->
+    io:format("Open~n", []).
+    ]]></code>
+    <p>
+      The selective receive in this case causes <c>open</c>
+      to implicitly postpone any events to the <c>locked</c> state.
+    </p>
+    <p>
+      A selective receive can not be used from a <c>gen_statem</c>
+      behaviour just as for any <c>gen_*</c> behavior
+      since the receive statement is within the <c>gen_*</c> engine itself.
+      It has to be there because all
+      <seealso marker="stdlib:sys"><c>sys</c></seealso>
+      compatible behaviours must respond to system messages and therefore
+      do that in their engine receive loop,
+      passing non-system messages to the callback module.
+    </p>
+    <p>
+      The state transition
+      <seealso marker="stdlib:gen_statem#type-action">
+	action
+      </seealso>
+      <c>postpone</c> is designed to be able to model
+      selective receives.  A selective receive implicitly postpones
+      any not received events, but the <c>postpone</c>
+      state transition action explicitly postpones one received event.
+    </p>
+    <p>
+      Other than that both mechanisms have got the same theoretical
+      time and memory complexity, while the selective receive
+      language construct has got smaller constant factors.
+    </p>
+    </section>
+  </section>
+
+<!-- =================================================================== -->
+
+  <section>
+    <title>Self Generated Events</title>
+    <p>
+      It may be beneficial in some cases to be able to generate events
+      to your own state machine.
+      This can be done with the state transition
+      <seealso marker="stdlib:gen_statem#type-action">
+	action
+      </seealso>
+      <c>{next_event,EventType,EventContent}</c>.
+    </p>
+    <p>
+      You can generate events of any existing
+      <seealso marker="stdlib:gen_statem#type-action">
+	type,
+      </seealso>
+      but the <c>internal</c> type can only be generated through the
+      <c>next_event</c> action and hence can not come from an external source,
+      so you can be certain that an <c>internal</c> event is an event
+      from your state machine to itself.
+    </p>
+    <p>
+      One example of using self generated events may be when you have
+      a state machine specification that uses state entry actions.
+      That you could code using a dedicated function
+      to do the state transition.  But if you want that code to be
+      visible besides the other state logic you can insert
+      an <c>internal</c> event that does the entry actions.
+      This has the same unfortunate consequence as using
+      state transition functions that everywhere you go to
+      the state in question you will have to explicitly
+      insert the <c>internal</c> event
+      or use state transition function.
+    </p>
+    <p>
+      Here is an implementation of entry actions
+      using <c>internal</c> events with content <c>enter</c>
+      utilizing a helper function <c>enter/3</c> for state entry:
+    </p>
+    <code type="erl"><![CDATA[
+...
+-define(CALLBACK_MODE, state_functions).
+
+...
+
+init(Code) ->
+    process_flag(trap_exit, true),
+    Data = #{code => Code},
+    enter(?CALLBACK_MODE, locked, Data).
+
+...
+
+locked(internal, enter, _Data) ->
+    do_lock(),
+    {keep_state,Data#{remaining => Code}};
+locked(
+  cast, {button,Digit},
+  #{code := Code, remaining := Remaining} = Data) ->
+    case Remaining of
+        [Digit] ->
+            enter(next_state, open, Data);
+...
+
+open(internal, enter, _Data) ->
+    Tref = erlang:start_timer(10000, self(), lock),
+    do_unlock(),
+    {keep_state,Data#{timer => Tref}};
+open(info, {timeout,Tref,lock}, #{timer := Tref} = Data) ->
+    enter(next_state, locked, Data);
+...
+
+enter(Tag, State, Data) ->
+    {Tag,State,Data,[{next_event,internal,enter}]}.
+    ]]></code>
+  </section>
+
+<!-- =================================================================== -->
+
+  <section>
+    <title>Example Revisited</title>
+    <p>
+      Here is the example after all mentioned modifications
+      and some more utilizing the entry actions,
+      which deserves a new state diagram:
+    </p>
+    <image file="../design_principles/code_lock_2.png">
+      <icaption>Code lock state diagram revisited</icaption>
+    </image>
+    <p>
+      Note that this state diagram does not specify how to handle
+      a button event in the state <c>open</c>, so you will have to
+      read some other place that is here that unspecified events
+      shall be ignored as in not consumed but handled in some other state.
+      Nor does it show that the <c>code_length/0</c> call shall be
+      handled in every state.
+    </p>
+
+    <section>
+      <title>Callback Mode: state_functions</title>
+      <p>
+	Using state functions:
+      </p>
+      <code type="erl"><![CDATA[
+-module(code_lock).
+-behaviour(gen_statem).
+-define(NAME, code_lock_2).
+-define(CALLBACK_MODE, state_functions).
+
+-export([start_link/1,stop/0]).
+-export([button/1,code_length/0]).
+-export([init/1,terminate/3,code_change/4]).
+-export([locked/3,open/3]).
+
+start_link(Code) ->
+    gen_statem:start_link({local,?NAME}, ?MODULE, Code, []).
+stop() ->
+    gen_statem:stop(?NAME).
+
+button(Digit) ->
+    gen_statem:cast(?NAME, {button,Digit}).
+code_length() ->
+    gen_statem:call(?NAME, code_length).
+
+init(Code) ->
+    process_flag(trap_exit, true),
+    Data = #{code => Code},
+    enter(?CALLBACK_MODE, locked, Data).
+
+locked(internal, enter, #{code := Code} = Data) ->
+    do_lock(),
+    {keep_state,Data#{remaining => Code}};
+locked(
+  cast, {button,Digit},
+  #{code := Code, remaining := Remaining} = Data) ->
+    case Remaining of
+        [Digit] -> % Complete
+            enter(next_state, open, Data);
+        [Digit|Rest] -> % Incomplete
+            {keep_state,Data#{remaining := Rest}};
+        [_|_] -> % Wrong
+            {keep_state,Data#{remaining := Code}}
+    end;
+locked(EventType, EventContent, Data) ->
+    handle_event(EventType, EventContent, Data).
+
+open(internal, enter, Data) ->
+    Tref = erlang:start_timer(10000, self(), lock),
+    do_unlock(),
+    {keep_state,Data#{timer => Tref}};
+open(info, {timeout,Tref,lock}, #{timer := Tref} = Data) ->
+    enter(next_state, locked, Data);
+open(cast, {button,_}, _) ->
+    {keep_state_and_data,[postpone]};
+open(EventType, EventContent, Data) ->
+    handle_event(EventType, EventContent, Data).
+
+handle_event({call,From}, code_length, #{code := Code}) ->
+    {keep_state_and_data,[{reply,From,length(Code)}]}.
+enter(Tag, State, Data) ->
+    {Tag,State,Data,[{next_event,internal,enter}]}.
+
+do_lock() ->
+    io:format("Locked~n", []).
+do_unlock() ->
+    io:format("Open~n", []).
+
+terminate(_Reason, State, _Data) ->
+    State =/= locked andalso do_lock(),
+    ok.
+code_change(_Vsn, State, Data, _Extra) ->
+    {?CALLBACK_MODE,State,Data}.
+      ]]></code>
+    </section>
+
+    <section>
+      <title>Callback Mode: handle_event_function</title>
+      <p>
+	What to change to use one <c>handle_event/4</c> function.
+	Here a clean first-dispatch-on-event approach
+	does not work that well due to the generated
+	entry actions:
+      </p>
+      <code type="erl"><![CDATA[
+...
+-define(CALLBACK_MODE, handle_event_function).
+
+...
+-export([handle_event/4]).
+
+...
+
+%% State: locked
+handle_event(internal, enter, locked, #{code := Code} = Data) ->
+    do_lock(),
+    {keep_state,Data#{remaining => Code}};
+handle_event(
+  cast, {button,Digit}, locked,
+  #{code := Code, remaining := Remaining} = Data) ->
+    case Remaining of
+        [Digit] -> % Complete
+            enter(next_state, open, Data);
+        [Digit|Rest] -> % Incomplete
+            {keep_state,Data#{remaining := Rest}};
+        [_|_] -> % Wrong
+            {keep_state,Data#{remaining := Code}}
+    end;
+%%
+%% State: open
+handle_event(internal, enter, open, Data) ->
+    Tref = erlang:start_timer(10000, self(), lock),
+    do_unlock(),
+    {keep_state,Data#{timer => Tref}};
+handle_event(info, {timeout,Tref,lock}, open, #{timer := Tref} = Data) ->
+    enter(next_state, locked, Data);
+handle_event(cast, {button,_}, open, _) ->
+    {keep_state_and_data,[postpone]};
+%%
+%% Any state
+handle_event({call,From}, code_length, _State, #{code := Code}) ->
+    {keep_state_and_data,[{reply,From,length(Code)}]}.
+
+...
+      ]]></code>
+    </section>
+    <p>
+      Note that postponing buttons from the <c>locked</c> state
+      to the <c>open</c> state feels like the wrong thing to do
+      for a code lock, but it at least illustrates event postponing.
+    </p>
+  </section>
+
+<!-- =================================================================== -->
+
+  <section>
+    <title>Filter the State</title>
+    <p>
+      The example servers so far in this chapter will for example
+      when killed by an exit signal or due to an internal error
+      print out the full internal state in the error log.
+      This state contains both the code lock code
+      and which digits that remains to unlock.
+    </p>
+    <p>
+      This state data can be regarded as sensitive,
+      and maybe not what you want in the error log
+      because of something unpredictable happening.
+    </p>
+    <p>
+      Another reason to filter the state can be
+      that the state is too big to print out since it fills
+      the error log with uninteresting details.
+    </p>
+    <p>
+      To avoid this you can format the internal state
+      that gets in the error log and gets returned from
+      <seealso marker="stdlib:sys#get_status/1">
+	<c>sys:get_status/1,2</c>
+      </seealso>
+      by implementing the
+      <seealso marker="stdlib:gen_statem#Module:format_status/2">
+	<c>Module:format_status/2</c>
+      </seealso>
+      function, for example like this:
+    </p>
+    <code type="erl"><![CDATA[
+...
+-export([init/1,terminate/3,code_change/4,format_status/2]).
+...
+
+format_status(Opt, [_PDict,State,Data]) ->
+    StateData =
+	{State,
+	 maps:filter(
+	   fun (code, _) -> false;
+	       (remaining, _) -> false;
+	       (_, _) -> true
+	   end,
+	   Data)},
+    case Opt of
+	terminate ->
+	    StateData;
+	normal ->
+	    [{data,[{"State",StateData}]}]
+    end.
+    ]]></code>
+    <p>
+      It is not mandatory to implement a
+      <seealso marker="stdlib:gen_statem#Module:format_status/2">
+	<c>Module:format_status/2</c>
+      </seealso>
+      function.  If you do not a default implementation is used that
+      does the same as this example function without filtering
+      the <c>Data</c> term that is: <c>StateData = {State,Data}</c>.
+    </p>
+  </section>
+
+<!-- =================================================================== -->
+
+  <section>
+    <title>Complex State</title>
+    <p>
+      The callback mode
+      <seealso marker="stdlib:gen_statem#type-callback_mode">
+	<c>handle_event_function</c>
+      </seealso>
+      enables using a non-atom state as described in
+      <seealso marker="#callback_modes">
+	Callback Modes,
+      </seealso>
+      for example a complex state term like a tuple.
+    </p>
+    <p>
+      One reason to use this is when you have
+      a state item that affects the event handling
+      in particular when combining that with postponing events.
+      Let us complicate the previous example
+      by introducing a configurable lock button
+      (this is the state item in question)
+      that in the <c>open</c> state immediately locks the door,
+      and an API function <c>set_lock_button/1</c> to set the lock button.
+    </p>
+    <p>
+      Suppose now that we call <c>set_lock_button</c>
+      while the door is open,
+      and have already postponed a button event
+      that up until now was not the lock button;
+      the sensible thing might be to say that
+      the button was pressed too early so it should
+      not be recognized as the lock button,
+      but then it might be surprising that a button event
+      that now is the lock button event arrives (as retried postponed)
+      immediately after the state transits to <c>locked</c>.
+    </p>
+    <p>
+      So let us make the <c>button/1</c> function synchronous
+      by using <c>gen_statem:call</c>,
+      and still postpone its events in the <c>open</c> state.
+      Then a call to <c>button/1</c> during the <c>open</c>
+      state will not return until the state transits to <c>locked</c>
+      since it is there the event is handled and the reply is sent.
+    </p>
+    <p>
+      If now one process calls <c>set_lock_button/1</c>
+      to change the lock button while some other process
+      hangs in <c>button/1</c> with the new lock button
+      it could be expected that the hanging lock button call
+      immediately takes effect and locks the lock.
+      Therefore we make the current lock button a part of the state
+      so when we change the lock button the state will change
+      and all postponed events will be retried.
+    </p>
+    <p>
+      We define the state as <c>{StateName,LockButton}</c>
+      where <c>StateName</c> is as before
+      and <c>LockButton</c> is the current lock button:
+    </p>
+    <code type="erl"><![CDATA[
+-module(code_lock).
+-behaviour(gen_statem).
+-define(NAME, code_lock_3).
+-define(CALLBACK_MODE, handle_event_function).
+
+-export([start_link/2,stop/0]).
+-export([button/1,code_length/0,set_lock_button/1]).
+-export([init/1,terminate/3,code_change/4,format_status/2]).
+-export([handle_event/4]).
+
+start_link(Code, LockButton) ->
+    gen_statem:start_link(
+        {local,?NAME}, ?MODULE, {Code,LockButton}, []).
+stop() ->
+    gen_statem:stop(?NAME).
+
+button(Digit) ->
+    gen_statem:call(?NAME, {button,Digit}).
+code_length() ->
+    gen_statem:call(?NAME, code_length).
+set_lock_button(LockButton) ->
+    gen_statem:call(?NAME, {set_lock_button,LockButton}).
+
+init({Code,LockButton}) ->
+    process_flag(trap_exit, true),
+    Data = #{code => Code, remaining => undefined, timer => undefined},
+    enter(?CALLBACK_MODE, {locked,LockButton}, Data, []).
+
+handle_event(
+  {call,From}, {set_lock_button,NewLockButton},
+  {StateName,OldLockButton}, Data) ->
+    {next_state,{StateName,NewLockButton},Data,
+     [{reply,From,OldLockButton}]};
+handle_event(
+  {call,From}, code_length,
+  {_StateName,_LockButton}, #{code := Code}) ->
+    {keep_state_and_data,
+     [{reply,From,length(Code)}]};
+handle_event(
+  EventType, EventContent,
+  {locked,LockButton}, #{code := Code, remaining := Remaining} = Data) ->
+    case {EventType,EventContent} of
+	{internal,enter} ->
+	    do_lock(),
+	    {keep_state,Data#{remaining := Code}};
+	{{call,From},{button,Digit}} ->
+	    case Remaining of
+		[Digit] -> % Complete
+		    next_state(
+		      {open,LockButton}, Data,
+		      [{reply,From,ok}]);
+		[Digit|Rest] -> % Incomplete
+		    {keep_state,Data#{remaining := Rest},
+		     [{reply,From,ok}]};
+		[_|_] -> % Wrong
+		    {keep_state,Data#{remaining := Code},
+		     [{reply,From,ok}]}
+	    end
+    end;
+handle_event(
+  EventType, EventContent,
+  {open,LockButton}, #{timer := Timer} = Data) ->
+    case {EventType,EventContent} of
+	{internal,enter} ->
+	    Tref = erlang:start_timer(10000, self(), lock),
+	    do_unlock(),
+	    {keep_state,Data#{timer := Tref}};
+	{info,{timeout,Timer,lock}} ->
+	    next_state({locked,LockButton}, Data, []);
+	{{call,From},{button,Digit}} ->
+	    if
+		Digit =:= LockButton ->
+		    erlang:cancel_timer(Timer),
+		    next_state(
+		      {locked,LockButton}, Data,
+		      [{reply,From,locked}]);
+		true ->
+		    {keep_state_and_data,
+		     [postpone]}
+	    end
+    end.
+
+next_state(State, Data, Actions) ->
+    enter(next_state, State, Data, Actions).
+enter(Tag, State, Data, Actions) ->
+    {Tag,State,Data,[{next_event,internal,enter}|Actions]}.
+
+do_lock() ->
+    io:format("Locked~n", []).
+do_unlock() ->
+    io:format("Open~n", []).
+
+terminate(_Reason, State, _Data) ->
+    State =/= locked andalso do_lock(),
+    ok.
+code_change(_Vsn, State, Data, _Extra) ->
+    {?CALLBACK_MODE,State,Data}.
+format_status(Opt, [_PDict,State,Data]) ->
+    StateData =
+	{State,
+	 maps:filter(
+	   fun (code, _) -> false;
+	       (remaining, _) -> false;
+	       (_, _) -> true
+	   end,
+	   Data)},
+    case Opt of
+	terminate ->
+	    StateData;
+	normal ->
+	    [{data,[{"State",StateData}]}]
+    end.
+    ]]></code>
+    <p>
+      It may be an ill-fitting model for a physical code lock
+      that the <c>button/1</c> call might hang until the lock
+      is locked.  But for an API in general it is really not
+      that strange.
+    </p>
+  </section>
+
+</chapter>
diff --git a/system/doc/design_principles/sup_princ.xml b/system/doc/design_principles/sup_princ.xml
index 7408a34442..a77b3964fc 100644
--- a/system/doc/design_principles/sup_princ.xml
+++ b/system/doc/design_principles/sup_princ.xml
@@ -213,6 +213,7 @@ child_spec() = #{id => child_id(),       % mandatory
 	  <item><c>supervisor:start_link</c></item>
 	  <item><c>gen_server:start_link</c></item>
 	  <item><c>gen_fsm:start_link</c></item>
+	  <item><c>gen_statem:start_link</c></item>
 	  <item><c>gen_event:start_link</c></item>
 	  <item>A function compliant with these functions. For details,
 	  see the <c>supervisor(3)</c> manual page.</item>
@@ -276,7 +277,8 @@ child_spec() = #{id => child_id(),       % mandatory
         <p><c>modules</c> are to be a list with one element
           <c>[Module]</c>, where <c>Module</c> is the name of
           the callback module, if the child process is a supervisor,
-          gen_server or gen_fsm. If the child process is a gen_event,
+          gen_server, gen_fsm or gen_statem.
+	  If the child process is a gen_event,
           the value shall be <c>dynamic</c>.</p>
         <p>This information is used by the release handler during
           upgrades and downgrades, see
@@ -400,8 +402,8 @@ supervisor:delete_child(Sup, Id)</code>
       restarts.</p>
   </section>
 
-  <marker id="simple"/>
   <section>
+    <marker id="simple"/>
     <title>Simplified one_for_one Supervisors</title>
     <p>A supervisor with restart strategy <c>simple_one_for_one</c> is
       a simplified <c>one_for_one</c> supervisor, where all child
diff --git a/system/doc/design_principles/xmlfiles.mk b/system/doc/design_principles/xmlfiles.mk
index 9c3836c8ac..e476255d62 100644
--- a/system/doc/design_principles/xmlfiles.mk
+++ b/system/doc/design_principles/xmlfiles.mk
@@ -25,6 +25,7 @@ DESIGN_PRINCIPLES_CHAPTER_FILES = \
 	distributed_applications.xml \
 	events.xml \
 	fsm.xml \
+	statem.xml \
 	gen_server_concepts.xml \
 	included_applications.xml \
 	release_handling.xml \
diff --git a/system/doc/reference_manual/modules.xml b/system/doc/reference_manual/modules.xml
index 5f2ac2a67d..96968b547e 100644
--- a/system/doc/reference_manual/modules.xml
+++ b/system/doc/reference_manual/modules.xml
@@ -4,7 +4,7 @@
 <chapter>
   <header>
     <copyright>
-      <year>2003</year><year>2015</year>
+      <year>2003</year><year>2016</year>
       <holder>Ericsson AB. All Rights Reserved.</holder>
     </copyright>
     <legalnotice>
@@ -144,6 +144,7 @@ fact(0) ->           %  |
 	<list type="bulleted">
 	  <item><c>gen_server</c></item>
 	  <item><c>gen_fsm</c></item>
+	  <item><c>gen_statem</c></item>
 	  <item><c>gen_event</c></item>
 	  <item><c>supervisor</c></item>
 	</list>