Include Merl in Syntax Tools

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diff --git a/lib/syntax_tools/src/merl.erl b/lib/syntax_tools/src/merl.erl
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+%% ---------------------------------------------------------------------
+%% 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.
+%%
+%% Note: EDoc uses @@ and @} as escape sequences, so in the doc text below,
+%% `@@' must be written `@@@@' and `@}' must be written `@@}'.
+%%
+%% @author Richard Carlsson <[email protected]>
+%% @copyright 2010-2015 Richard Carlsson
+%%
+%% @doc Metaprogramming in Erlang.
+%% Merl is a more user friendly interface to the `erl_syntax' module, making
+%% it easy both to build new ASTs from scratch and to
+%% match and decompose existing ASTs. For details that are outside the scope
+%% of Merl itself, please see the documentation of {@link erl_syntax}.
+%%
+%% == Quick start ==
+%%
+%% To enable the full power of Merl, your module needs to include the Merl
+%% header file:
+%% ```-include_lib("syntax_tools/include/merl.hrl").'''
+%%
+%% Then, you can use the `?Q(Text)' macros in your code to create ASTs or match
+%% on existing ASTs. For example:
+%% ```Tuple = ?Q("{foo, 42}"),
+%%    ?Q("{foo, _@Number}") = Tuple,
+%%    Call = ?Q("foo:bar(_@Number)")'''
+%%
+%% Calling `merl:print(Call)' will then print the following code:
+%% ```foo:bar(42)'''
+%%
+%% The `?Q' macros turn the quoted code fragments into ASTs, and lifts
+%% metavariables such as `_@Tuple' and `_@Number' to the level of your Erlang
+%% code, so you can use the corresponding Erlang variables `Tuple' and `Number'
+%% directly. This is the most straightforward way to use Merl, and in many
+%% cases it's all you need.
+%%
+%% You can even write case switches using `?Q' macros as patterns. For example:
+%% ```case AST of
+%%        ?Q("{foo, _@Foo}") -> handle(Foo);
+%%        ?Q("{bar, _@Bar}") when erl_syntax:is_integer(Bar) -> handle(Bar);
+%%        _ -> handle_default()
+%%    end'''
+%%
+%% These case switches only allow `?Q(...)' or `_' as clause patterns, and the
+%% guards may contain any expressions, not just Erlang guard expressions.
+%%
+%% If the macro `MERL_NO_TRANSFORM' is defined before the `merl.hrl' header
+%% file is included, the parse transform used by Merl will be disabled, and in
+%% that case, the match expressions `?Q(...) = ...', case switches using
+%% `?Q(...)' patterns, and automatic metavariables like `_@Tuple' cannot be
+%% used in your code, but the Merl macros and functions still work. To do
+%% metavariable substitution, you need to use the `?Q(Text, Map)' macro, e.g.:
+%% ```Tuple = ?Q("{foo, _@bar, _@baz}", [{bar, Bar}, {baz,Baz}])'''
+%%
+%% The text given to a `?Q(Text)' macro can be either a single string, or a
+%% list of strings. The latter is useful when you need to split a long
+%% expression over multiple lines, e.g.:
+%% ```?Q(["case _@Expr of",
+%%        "  {foo, X} -> f(X);",
+%%        "  {bar, X} -> g(X)",
+%%        "  _ -> h(X)"
+%%        "end"])'''
+%% If there is a syntax error somewhere in the text (like the missing semicolon
+%% in the second clause above) this allows Merl to generate an error message
+%% pointing to the exact line in your source code. (Just remember to
+%% comma-separate the strings in the list, otherwise Erlang will concatenate
+%% the string fragments as if they were a single string.)
+%%
+%% == Metavariable syntax ==
+%%
+%% There are several ways to write a metavariable in your quoted code:
+%% <ul>
+%%   <li>Atoms starting with `@', for example `` '@foo' '' or `` '@Foo' ''</li>
+%%   <li>Variables starting with `_@', for example `_@bar' or `_@Bar'</li>
+%%   <li>Strings starting with ``"'@'', for example ``"'@File"''</li>
+%%   <li>Integers starting with 909, for example `9091' or `909123'</li>
+%% </ul>
+%% Following the prefix, one or more `_' or `0' characters may be used to
+%% indicate "lifting" of the variable one or more levels, and after that, a `@'
+%% or `9' character indicates a glob metavariable (matching zero or more
+%% elements in a sequence) rather than a normal metavariable. For example:
+%% <ul>
+%%   <li>`` '@_foo' '' is lifted one level, and `_@__foo' is lifted two
+%%       levels</li>
+%%   <li>`_@@@@bar' is a glob variable, and `_@_@bar' is a lifted glob
+%%       variable</li>
+%%   <li>`90901' is a lifted variable,`90991' is a glob variable, and `9090091'
+%%       is a glob variable lifted two levels</li>
+%% </ul>
+%% (Note that the last character in the name is never considered to be a lift
+%% or glob marker, hence, `_@__' and `90900' are only lifted one level, not
+%% two. Also note that globs only matter for matching; when doing
+%% substitutions, a non-glob variable can be used to inject a sequence of
+%% elements, and vice versa.)
+%%
+%% If the name after the prefix and any lift and glob markers is `_' or `0',
+%% the variable is treated as an anonymous catch-all pattern in matches. For
+%% example, `_@_', `_@@@@_', `_@__', or even `_@__@_'.
+%%
+%% Finally, if the name without any prefixes or lift/glob markers begins with
+%% an uppercase character, as in `_@Foo' or `_@_@Foo', it will become a
+%% variable on the Erlang level, and can be used to easily deconstruct and
+%% construct syntax trees:
+%% ```case Input of
+%%        ?Q("{foo, _@Number}") -> ?Q("foo:bar(_@Number)");
+%%        ...'''
+%% We refer to these as "automatic metavariables". If in addition the name ends
+%% with `@', as in `_@Foo@', the value of the variable as an Erlang term will
+%% be automatically converted to the corresponding abstract syntax tree when
+%% used to construct a larger tree. For example, in:
+%% ```Bar = {bar, 42},
+%%    Foo = ?Q("{foo, _@Bar@@}")'''
+%% (where Bar is just some term, not a syntax tree) the result `Foo' will be a
+%% syntax tree representing `{foo, {bar, 42}}'. This avoids the need for
+%% temporary variables in order to inject data, as in
+%% ```TmpBar = erl_syntax:abstract(Bar),
+%%    Foo = ?Q("{foo, _@TmpBar}")'''
+%%
+%% If the context requires an integer rather than a variable, an atom, or a
+%% string, you cannot use the uppercase convention to mark an automatic
+%% metavariable. Instead, if the integer (without the `909'-prefix and
+%% lift/glob markers) ends in a `9', the integer will become an Erlang-level
+%% variable prefixed with `Q', and if it ends with `99' it will also be
+%% automatically abstracted. For example, the following will increment the
+%% arity of the exported function f:
+%% ```case Form of
+%%        ?Q("-export([f/90919]).") ->
+%%            Q2 = erl_syntax:concrete(Q1) + 1,
+%%            ?Q("-export([f/909299]).");
+%%        ...'''
+%%
+%% == When to use the various forms of metavariables ==
+%%
+%% Merl can only parse a fragment of text if it follows the basic syntactical
+%% rules of Erlang. In most places, a normal Erlang variable can be used as
+%% metavariable, for example:
+%% ```?Q("f(_@Arg)") = Expr'''
+%% but if you want to match on something like the name of a function, you have
+%% to use an atom as metavariable:
+%% ```?Q("'@Name'() -> _@@@@_." = Function'''
+%% (note the anonymous glob variable `_@@@@_' to ignore the function body).
+%%
+%% In some contexts, only a string or an integer is allowed. For example, the
+%% directive `-file(Name, Line)' requires that `Name' is a string literal and
+%% `Line' an integer literal:
+%%
+%% ```?Q("-file(\"'@File\", 9090).") = ?Q("-file(\"foo.erl\", 42).")).'''
+%% This will extract the string literal `"foo.erl"' into the variable `Foo'.
+%% Note the use of the anonymous variable `9090' to ignore the line number. To
+%% match and also bind a metavariable that must be an integer literal, we can
+%% use the convention of ending the integer with a 9, turning it into a
+%% Q-prefixed variable on the Erlang level (see the previous section).
+%%
+%% === Globs ===
+%%
+%% Whenever you want to match out a number of elements in a sequence (zero or
+%% more) rather than a fixed set of elements, you need to use a glob. For
+%% example:
+%% ```?Q("{_@@@@Elements}") = ?Q({a, b, c})'''
+%% will bind Elements to the list of individual syntax trees representing the
+%% atoms `a', `b', and `c'. This can also be used with static prefix and suffix
+%% elements in the sequence. For example:
+%% ```?Q("{a, b, _@@@@Elements}") = ?Q({a, b, c, d})'''
+%% will bind Elements to the list of the `c' and `d' subtrees, and
+%% ```?Q("{_@@@@Elements, c, d}") = ?Q({a, b, c, d})'''
+%% will bind Elements to the list of the `a' and `b' subtrees. You can even use
+%% plain metavariables in the prefix or suffix:
+%% ```?Q("{_@First, _@@@@Rest}") = ?Q({a, b, c})'''
+%% or
+%% ```?Q("{_@@@@_, _@Last}") = ?Q({a, b, c})'''
+%% (ignoring all but the last element). You cannot however have two globs as
+%% part of the same sequence.
+%%
+%% === Lifted metavariables ===
+%%
+%% In some cases, the Erlang syntax rules make it impossible to place a
+%% metavariable directly where you would like it. For example, you cannot
+%% write:
+%% ```?Q("-export([_@@@@Name]).")'''
+%% to match out all name/arity pairs in the export list, or to insert a list of
+%% exports in a declaration, because the Erlang parser only allows elements on
+%% the form `A/I' (where `A' is an atom and `I' an integer) in the export list.
+%% A variable like the above is not allowed, but neither is a single atom or
+%% integer, so `` '@@@@Name' '' or `909919' wouldn't work either.
+%%
+%% What you have to do in such cases is to write your metavariable in a
+%% syntactically valid position, and use lifting markers to denote where it
+%% should really apply, as in:
+%% ```?Q("-export(['@@_@@Name'/0]).")'''
+%% This causes the variable to be lifted (after parsing) to the next higher
+%% level in the syntax tree, replacing that entire subtree. In this case, the
+%% `` '@@_@@Name'/0 '' will be replaced with `` '@@@@Name' '', and the ``/0''
+%% part was just used as dummy notation and will be discarded.
+%%
+%% You may even need to apply lifting more than once. To match the entire
+%% export list as a single syntax tree, you can write:
+%% ```?Q("-export(['@@__Name'/0]).")'''
+%% using two underscores, but with no glob marker this time. This will make the
+%% entire ``['@@__Name'/0]'' part be replaced with `` '@@Name' ''.
+%%
+%% Sometimes, the tree structure of a code fragment isn't very obvious, and
+%% parts of the structure may be invisible when printed as source code. For
+%% instance, a simple function definition like the following:
+%% ```zero() -> 0.'''
+%% consists of the name (the atom `zero'), and a list of clauses containing the
+%% single clause `() -> 0'. The clause consists of an argument list (empty), a
+%% guard (empty), and a body (which is always a list of expressions) containing
+%% the single expression `0'. This means that to match out the name and the
+%% list of clauses of any function, you'll need to use a pattern like
+%% ``?Q("'@Name'() -> _@_@Body.")'', using a dummy clause whose body is a glob
+%% lifted one level.
+%%
+%% To visualize the structure of a syntax tree, you can use the function
+%% `merl:show(T)', which prints a summary. For example, entering
+%% ```merl:show(merl:quote("inc(X, Y) when Y > 0 -> X + Y."))'''
+%% in the Erlang shell will print the following (where the `+' signs separate
+%% groups of subtrees on the same level):
+%% ```function: inc(X, Y) when ... -> X + Y.
+%%      atom: inc
+%%      +
+%%      clause: (X, Y) when ... -> X + Y
+%%        variable: X
+%%        variable: Y
+%%        +
+%%        disjunction: Y > 0
+%%          conjunction: Y > 0
+%%            infix_expr: Y > 0
+%%              variable: Y
+%%              +
+%%              operator: >
+%%              +
+%%              integer: 0
+%%        +
+%%        infix_expr: X + Y
+%%          variable: X
+%%          +
+%%          operator: +
+%%          +
+%%          variable: Y'''
+%%
+%% This shows another important non-obvious case: a clause guard, even if it's
+%% as simple as `Y > 0', always consists of a single disjunction of one or more
+%% conjunctions of tests, much like a tuple of tuples. Thus:
+%% <ul>
+%%   <li>``"when _@Guard ->"'' will only match a guard with exactly one
+%%     test</li>
+%%   <li>``"when _@@@@Guard ->"'' will match a guard with one or more
+%%     comma-separated tests (but no semicolons), binding `Guard' to the list
+%%     of tests</li>
+%%   <li>``"when _@_Guard ->"'' will match just like the previous pattern, but
+%%     binds `Guard' to the conjunction subtree</li>
+%%   <li>``"when _@_@Guard ->"'' will match an arbitrary nonempty guard,
+%%     binding `Guard' to the list of conjunction subtrees</li>
+%%   <li>``"when _@__Guard ->"'' will match like the previous pattern, but
+%%     binds `Guard' to the whole disjunction subtree</li>
+%%   <li>and finally, ``"when _@__@Guard ->"'' will match any clause,
+%%     binding `Guard' to `[]' if the guard is empty and to `[Disjunction]'
+%%     otherwise</li>
+%% </ul>
+%%
+%% Thus, the following pattern matches all possible clauses:
+%% ```"(_@@Args) when _@__@Guard -> _@@Body"'''
+%% @end
+
+-module(merl).
+
+-export([term/1, var/1, print/1, show/1]).
+
+-export([quote/1, quote/2, qquote/2, qquote/3]).
+
+-export([template/1, tree/1, subst/2, tsubst/2, alpha/2, match/2, switch/2]).
+
+-export([template_vars/1, meta_template/1]).
+
+-export([compile/1, compile/2, compile_and_load/1, compile_and_load/2]).
+
+%% NOTE: this module may not include merl.hrl!
+
+-type tree() :: erl_syntax:syntaxTree().
+
+-type tree_or_trees() :: tree() | [tree()].
+
+-type pattern() :: tree() | template().
+
+-type pattern_or_patterns() :: pattern() | [pattern()].
+
+-type env() :: [{Key::id(), pattern_or_patterns()}].
+
+-type id() :: atom() | integer().
+
+%% A list of strings or binaries is assumed to represent individual lines,
+%% while a flat string or binary represents source code containing newlines.
+-type text() :: string() | binary() | [string()] | [binary()].
+
+-type location() :: erl_scan:location().
+
+
+%% ------------------------------------------------------------------------
+%% Compiling and loading code directly to memory
+
+%% @equiv compile(Code, [])
+compile(Code) ->
+    compile(Code, []).
+
+%% @doc Compile a syntax tree or list of syntax trees representing a module
+%% into a binary BEAM object.
+%% @see compile_and_load/2
+%% @see compile/1
+compile(Code, Options) when not is_list(Code)->
+    case type(Code) of
+        form_list -> compile(erl_syntax:form_list_elements(Code));
+        _ -> compile([Code], Options)
+    end;
+compile(Code, Options0) when is_list(Options0) ->
+    Forms = [erl_syntax:revert(F) || F <- Code],
+    Options = [verbose, report_errors, report_warnings, binary | Options0],
+    compile:noenv_forms(Forms, Options).
+
+
+%% @equiv compile_and_load(Code, [])
+compile_and_load(Code) ->
+    compile_and_load(Code, []).
+
+%% @doc Compile a syntax tree or list of syntax trees representing a module
+%% and load the resulting module into memory.
+%% @see compile/2
+%% @see compile_and_load/1
+compile_and_load(Code, Options) ->
+    case compile(Code, Options) of
+        {ok, ModuleName, Binary} ->
+            _ = code:load_binary(ModuleName, "", Binary),
+            {ok, Binary};
+        Other -> Other
+    end.
+
+
+%% ------------------------------------------------------------------------
+%% Utility functions
+
+
+-spec var(atom()) -> tree().
+
+%% @doc Create a variable.
+
+var(Name) ->
+    erl_syntax:variable(Name).
+
+
+-spec term(term()) -> tree().
+
+%% @doc Create a syntax tree for a constant term.
+
+term(Term) ->
+    erl_syntax:abstract(Term).
+
+
+%% @doc Pretty-print a syntax tree or template to the standard output. This
+%% is a utility function for development and debugging.
+
+print(Ts) when is_list(Ts) ->
+    lists:foreach(fun print/1, Ts);
+print(T) ->
+    io:put_chars(erl_prettypr:format(tree(T))),
+    io:nl().
+
+%% @doc Print the structure of a syntax tree or template to the standard
+%% output. This is a utility function for development and debugging.
+
+show(Ts) when is_list(Ts) ->
+    lists:foreach(fun show/1, Ts);
+show(T) ->
+    io:put_chars(pp(tree(T), 0)),
+    io:nl().
+
+pp(T, I) ->
+    [lists:duplicate(I, $\s),
+     limit(lists:flatten([atom_to_list(type(T)), ": ",
+                          erl_prettypr:format(erl_syntax_lib:limit(T,3))]),
+           79-I),
+     $\n,
+     pp_1(lists:filter(fun (X) -> X =/= [] end, subtrees(T)), I+2)
+    ].
+
+pp_1([G], I) ->
+    pp_2(G, I);
+pp_1([G | Gs], I) ->
+    [pp_2(G, I), lists:duplicate(I, $\s), "+\n" | pp_1(Gs, I)];
+pp_1([], _I) ->
+    [].
+
+pp_2(G, I) ->
+    [pp(E, I) || E <- G].
+
+%% limit string to N characters, stay on a single line and compact whitespace
+limit([$\n | Cs], N) -> limit([$\s | Cs], N);
+limit([$\r | Cs], N) -> limit([$\s | Cs], N);
+limit([$\v | Cs], N) -> limit([$\s | Cs], N);
+limit([$\t | Cs], N) -> limit([$\s | Cs], N);
+limit([$\s, $\s | Cs], N) -> limit([$\s | Cs], N);
+limit([C | Cs], N) when C < 32 -> limit(Cs, N);
+limit([C | Cs], N) when N > 3 -> [C | limit(Cs, N-1)];
+limit([_C1, _C2, _C3, _C4 | _Cs], 3) -> "...";
+limit(Cs, 3) -> Cs;
+limit([_C1, _C2, _C3 | _], 2) -> "..";
+limit(Cs, 2) -> Cs;
+limit([_C1, _C2 | _], 1) -> ".";
+limit(Cs, 1) -> Cs;
+limit(_, _) -> [].
+
+%% ------------------------------------------------------------------------
+%% Parsing and instantiating code fragments
+
+
+-spec qquote(Text::text(), Env::env()) -> tree_or_trees().
+
+%% @doc Parse text and substitute meta-variables.
+%%
+%% @equiv qquote(1, Text, Env)
+
+qquote(Text, Env) ->
+    qquote(1, Text, Env).
+
+
+-spec qquote(StartPos::location(), Text::text(), Env::env()) -> tree_or_trees().
+
+%% @doc Parse text and substitute meta-variables. Takes an initial scanner
+%% starting position as first argument.
+%%
+%% The macro `?Q(Text, Env)' expands to `merl:qquote(?LINE, Text, Env)'.
+%%
+%% @see quote/2
+
+qquote(StartPos, Text, Env) ->
+    subst(quote(StartPos, Text), Env).
+
+
+-spec quote(Text::text()) -> tree_or_trees().
+
+%% @doc Parse text.
+%%
+%% @equiv quote(1, Text)
+
+quote(Text) ->
+    quote(1, Text).
+
+
+-spec quote(StartPos::location(), Text::text()) -> tree_or_trees().
+
+%% @doc Parse text. Takes an initial scanner starting position as first
+%% argument.
+%%
+%% The macro `?Q(Text)' expands to `merl:quote(?LINE, Text, Env)'.
+%%
+%% @see quote/1
+
+quote({Line, Col}, Text)
+  when is_integer(Line), is_integer(Col) ->
+    quote_1(Line, Col, Text);
+quote(StartPos, Text) when is_integer(StartPos) ->
+    quote_1(StartPos, undefined, Text).
+
+quote_1(StartLine, StartCol, Text) ->
+    %% be backwards compatible as far as R12, ignoring any starting column
+    StartPos = case erlang:system_info(version) of
+                   "5.6" ++ _ -> StartLine;
+                   "5.7" ++ _ -> StartLine;
+                   "5.8" ++ _ -> StartLine;
+                   _ when StartCol =:= undefined -> StartLine;
+                   _ -> {StartLine, StartCol}
+               end,
+    FlatText = flatten_text(Text),
+    {ok, Ts, _} = erl_scan:string(FlatText, StartPos),
+    merge_comments(StartLine, erl_comment_scan:string(FlatText), parse_1(Ts)).
+
+parse_1(Ts) ->
+    %% if dot tokens are present, it is assumed that the text represents
+    %% complete forms, not dot-terminated expressions or similar
+    case split_forms(Ts) of
+        {ok, Fs} -> parse_forms(Fs);
+        error ->
+            parse_2(Ts)
+    end.
+
+split_forms(Ts) ->
+    split_forms(Ts, [], []).
+
+split_forms([{dot,_}=T|Ts], Fs, As) ->
+    split_forms(Ts, [lists:reverse(As, [T]) | Fs], []);
+split_forms([T|Ts], Fs, As) ->
+    split_forms(Ts, Fs, [T|As]);
+split_forms([], Fs, []) ->
+    {ok, lists:reverse(Fs)};
+split_forms([], [], _) ->
+    error;  % no dot tokens found - not representing form(s)
+split_forms([], _, [T|_]) ->
+    fail("incomplete form after ~p", [T]).
+
+parse_forms([Ts | Tss]) ->
+    case erl_parse:parse_form(Ts) of
+        {ok, Form} -> [Form | parse_forms(Tss)];
+        {error, R} -> parse_error(R)
+    end;
+parse_forms([]) ->
+    [].
+
+parse_2(Ts) ->
+    %% one or more comma-separated expressions?
+    %% (recall that Ts has no dot tokens if we get to this stage)
+    case erl_parse:parse_exprs(Ts ++ [{dot,0}]) of
+        {ok, Exprs} -> Exprs;
+        {error, E} ->
+            parse_3(Ts ++ [{'end',0}, {dot,0}], [E])
+    end.
+
+parse_3(Ts, Es) ->
+    %% try-clause or clauses?
+    case erl_parse:parse_exprs([{'try',0}, {atom,0,true}, {'catch',0} | Ts]) of
+        {ok, [{'try',_,_,_,_,_}=X]} ->
+            %% get the right kind of qualifiers in the clause patterns
+            erl_syntax:try_expr_handlers(X);
+        {error, E} ->
+            parse_4(Ts, [E|Es])
+    end.
+
+parse_4(Ts, Es) ->
+    %% fun-clause or clauses? (`(a)' is also a pattern, but `(a,b)' isn't,
+    %% so fun-clauses must be tried before normal case-clauses
+    case erl_parse:parse_exprs([{'fun',0} | Ts]) of
+        {ok, [{'fun',_,{clauses,Cs}}]} -> Cs;
+        {error, E} ->
+            parse_5(Ts, [E|Es])
+    end.
+
+parse_5(Ts, Es) ->
+    %% case-clause or clauses?
+    case erl_parse:parse_exprs([{'case',0}, {atom,0,true}, {'of',0} | Ts]) of
+        {ok, [{'case',_,_,Cs}]} -> Cs;
+        {error, E} ->
+            %% select the best error to report
+            parse_error(lists:last(lists:sort([E|Es])))
+    end.
+
+-dialyzer({nowarn_function, parse_error/1}). % no local return
+
+parse_error({L, M, R}) when is_atom(M), is_integer(L) ->
+    fail("~w: ~s", [L, M:format_error(R)]);
+parse_error({{L,C}, M, R}) when is_atom(M), is_integer(L), is_integer(C) ->
+    fail("~w:~w: ~s", [L,C,M:format_error(R)]);
+parse_error({_, M, R}) when is_atom(M) ->
+    fail(M:format_error(R));
+parse_error(R) ->
+    fail("unknown parse error: ~p", [R]).
+
+%% ------------------------------------------------------------------------
+%% Templates, substitution and matching
+
+%% Leaves are normal syntax trees, and inner nodes are tuples
+%% {template,Type,Attrs,Groups} where Groups are lists of lists of nodes.
+%% Metavariables are 1-tuples {VarName}, where VarName is an atom or an
+%% integer. {'_'} and {0} work as anonymous variables in matching. Glob
+%% metavariables are tuples {'*',VarName}, and {'*','_'} and {'*',0} are
+%% anonymous globs.
+
+%% Note that although template() :: tree() | ..., it is implied that these
+%% syntax trees are free from metavariables, so pattern() :: tree() |
+%% template() is in fact a wider type than template().
+
+-type template() :: tree()
+                  | {id()}
+                  | {'*',id()}
+                  | {template, atom(), term(), [[template()]]}.
+
+-type template_or_templates() :: template() | [template()].
+
+-spec template(pattern_or_patterns()) -> template_or_templates().
+
+%% @doc Turn a syntax tree or list of trees into a template or templates.
+%% Templates can be instantiated or matched against, and reverted back to
+%% normal syntax trees using {@link tree/1}. If the input is already a
+%% template, it is not modified further.
+%%
+%% @see subst/2
+%% @see match/2
+%% @see tree/1
+
+template(Trees) when is_list(Trees) ->
+    [template_0(T) || T <- Trees];
+template(Tree) ->
+    template_0(Tree).
+
+template_0({template, _, _, _}=Template) -> Template;
+template_0({'*',_}=Template) -> Template;
+template_0({_}=Template) -> Template;
+template_0(Tree) ->
+    case template_1(Tree) of
+        false -> Tree;
+        {Name} when is_list(Name) ->
+            fail("bad metavariable: '~s'", [tl(Name)]);  % drop v/n from name
+        Template -> Template
+    end.
+
+%% returns either a template or a lifted metavariable {String}, or 'false'
+%% if Tree contained no metavariables
+template_1(Tree) ->
+    case subtrees(Tree) of
+        [] ->
+            case metavar(Tree) of
+                {"v_"++Cs}=V when Cs =/= [] -> V;  % to be lifted
+                {"n0"++Cs}=V when Cs =/= [] -> V;  % to be lifted
+                {"v@"++Cs} when Cs =/= [] -> {'*',list_to_atom(Cs)};
+                {"n9"++Cs} when Cs =/= [] -> {'*',list_to_integer(Cs)};
+                {"v"++Cs} -> {list_to_atom(Cs)};
+                {"n"++Cs} -> {list_to_integer(Cs)};
+                false -> false
+            end;
+        Gs ->
+            case template_2(Gs, [], false) of
+                Gs1 when is_list(Gs1) ->
+                    {template, type(Tree), erl_syntax:get_attrs(Tree), Gs1};
+                Other ->
+                    Other
+            end
+    end.
+
+template_2([G | Gs], As, Bool) ->
+    case template_3(G, [], false) of
+        {"v_"++Cs}=V when Cs =/= [] -> V;  % lift further
+        {"n0"++Cs}=V when Cs =/= [] -> V;  % lift further
+        {"v@"++Cs} when Cs =/= [] -> {'*',list_to_atom(Cs)};  % stop
+        {"n9"++Cs} when Cs =/= [] -> {'*',list_to_integer(Cs)};  % stop
+        {"v"++Cs} when is_list(Cs) -> {list_to_atom(Cs)};  % stop
+        {"n"++Cs} when is_list(Cs) -> {list_to_integer(Cs)};  % stop
+        false -> template_2(Gs, [G | As], Bool);
+        G1 -> template_2(Gs, [G1 | As], true)
+    end;
+template_2([], _As, false) -> false;
+template_2([], As, true) -> lists:reverse(As).
+
+template_3([T | Ts], As, Bool) ->
+    case template_1(T) of
+        {"v_"++Cs} when Cs =/= [] -> {"v"++Cs};  % lift
+        {"n0"++Cs} when Cs =/= [] -> {"n"++Cs};  % lift
+        false -> template_3(Ts, [T | As], Bool);
+        T1 -> template_3(Ts, [T1 | As], true)
+    end;
+template_3([], _As, false) -> false;
+template_3([], As, true) -> lists:reverse(As).
+
+
+%% @doc Turn a template into a syntax tree representing the template.
+%% Meta-variables in the template are turned into normal Erlang variables if
+%% their names (after the metavariable prefix characters) begin with an
+%% uppercase character. E.g., `_@Foo' in the template becomes the variable
+%% `Foo' in the meta-template. Furthermore, variables ending with `@' are
+%% automatically wrapped in a call to merl:term/1, so e.g. `_@Foo@ in the
+%% template becomes `merl:term(Foo)' in the meta-template.
+
+-spec meta_template(template_or_templates()) -> tree_or_trees().
+
+meta_template(Templates) when is_list(Templates) ->
+    [meta_template_1(T) || T <- Templates];
+meta_template(Template) ->
+    meta_template_1(Template).
+
+meta_template_1({template, Type, Attrs, Groups}) ->
+    erl_syntax:tuple(
+      [erl_syntax:atom(template),
+       erl_syntax:atom(Type),
+       erl_syntax:abstract(Attrs),
+       erl_syntax:list([erl_syntax:list([meta_template_1(T) || T <- G])
+                        || G <- Groups])]);
+meta_template_1({Var}=V) ->
+    meta_template_2(Var, V);
+meta_template_1({'*',Var}=V) ->
+    meta_template_2(Var, V);
+meta_template_1(Leaf) ->
+    erl_syntax:abstract(Leaf).
+
+meta_template_2(Var, V) when is_atom(Var) ->
+    case atom_to_list(Var) of
+        [C|_]=Name when C >= $A, C =< $Z ; C >= $À, C =< $Þ, C /= $× ->
+            case lists:reverse(Name) of
+                "@"++([_|_]=RevRealName) ->  % don't allow empty RealName
+                    RealName = lists:reverse(RevRealName),
+                    erl_syntax:application(erl_syntax:atom(merl),
+                                           erl_syntax:atom(term),
+                                           [erl_syntax:variable(RealName)]);
+                _ ->
+                    %% plain automatic metavariable
+                    erl_syntax:variable(Name)
+            end;
+        _ ->
+            erl_syntax:abstract(V)
+    end;
+meta_template_2(Var, V) when is_integer(Var) ->
+    if Var > 9, (Var rem 10) =:= 9 ->
+            %% at least 2 digits, ends in 9: make it a Q-variable
+            if Var > 99, (Var rem 100) =:= 99 ->
+                    %% at least 3 digits, ends in 99: wrap in merl:term/1
+                    Name = "Q" ++ integer_to_list(Var div 100),
+                    erl_syntax:application(erl_syntax:atom(merl),
+                                           erl_syntax:atom(term),
+                                           [erl_syntax:variable(Name)]);
+               true ->
+                    %% plain automatic Q-variable
+                    Name = integer_to_list(Var div 10),
+                    erl_syntax:variable("Q" ++ Name)
+            end;
+       true ->
+            erl_syntax:abstract(V)
+    end.
+
+
+
+-spec template_vars(template_or_templates()) -> [id()].
+
+%% @doc Return an ordered list of the metavariables in the template.
+
+template_vars(Template) ->
+    template_vars(Template, []).
+
+template_vars(Templates, Vars) when is_list(Templates) ->
+    lists:foldl(fun template_vars_1/2, Vars, Templates);
+template_vars(Template, Vars) ->
+    template_vars_1(Template, Vars).
+
+template_vars_1({template, _, _, Groups}, Vars) ->
+    lists:foldl(fun (G, V) -> lists:foldl(fun template_vars_1/2, V, G) end,
+                Vars, Groups);
+template_vars_1({Var}, Vars) ->
+    ordsets:add_element(Var, Vars);
+template_vars_1({'*',Var}, Vars) ->
+    ordsets:add_element(Var, Vars);
+template_vars_1(_, Vars) ->
+    Vars.
+
+
+-spec tree(template_or_templates()) -> tree_or_trees().
+
+%% @doc Revert a template to a normal syntax tree. Any remaining
+%% metavariables are turned into `@'-prefixed atoms or `909'-prefixed
+%% integers.
+%% @see template/1
+
+tree(Templates) when is_list(Templates) ->
+    [tree_1(T) || T <- Templates];
+tree(Template) ->
+    tree_1(Template).
+
+tree_1({template, Type, Attrs, Groups}) ->
+    %% flattening here is needed for templates created via source transforms
+    Gs = [lists:flatten([tree_1(T) || T <- G]) || G <- Groups],
+    erl_syntax:set_attrs(make_tree(Type, Gs), Attrs);
+tree_1({Var}) when is_atom(Var) ->
+    erl_syntax:atom(list_to_atom("@"++atom_to_list(Var)));
+tree_1({Var}) when is_integer(Var) ->
+    erl_syntax:integer(list_to_integer("909"++integer_to_list(Var)));
+tree_1({'*',Var}) when is_atom(Var) ->
+    erl_syntax:atom(list_to_atom("@@"++atom_to_list(Var)));
+tree_1({'*',Var}) when is_integer(Var) ->
+    erl_syntax:integer(list_to_integer("9099"++integer_to_list(Var)));
+tree_1(Leaf) ->
+    Leaf.  % any syntax tree, not necessarily atomic (due to substitutions)
+
+
+-spec subst(pattern_or_patterns(), env()) -> tree_or_trees().
+
+%% @doc Substitute metavariables in a pattern or list of patterns, yielding
+%% a syntax tree or list of trees as result. Both for normal metavariables
+%% and glob metavariables, the substituted value may be a single element or
+%% a list of elements. For example, if a list representing `1, 2, 3' is
+%% substituted for `var' in either of `[foo, _@var, bar]' or `[foo, _@@var,
+%% bar]', the result represents `[foo, 1, 2, 3, bar]'.
+
+subst(Trees, Env) when is_list(Trees) ->
+    [subst_0(T, Env) || T <- Trees];
+subst(Tree, Env) ->
+    subst_0(Tree, Env).
+
+subst_0(Tree, Env) ->
+    tree_1(subst_1(template(Tree), Env)).
+
+
+-spec tsubst(pattern_or_patterns(), env()) -> template_or_templates().
+
+%% @doc Like subst/2, but does not convert the result from a template back
+%% to a tree. Useful if you want to do multiple separate substitutions.
+%% @see subst/2
+%% @see tree/1
+
+tsubst(Trees, Env) when is_list(Trees) ->
+    [subst_1(template(T), Env) || T <- Trees];
+tsubst(Tree, Env) ->
+    subst_1(template(Tree), Env).
+
+subst_1({template, Type, Attrs, Groups}, Env) ->
+    Gs1 = [lists:flatten([subst_1(T, Env) || T <- G]) || G <- Groups],
+    {template, Type, Attrs, Gs1};
+subst_1({Var}=V, Env) ->
+    case lists:keyfind(Var, 1, Env) of
+        {Var, TreeOrTrees} -> TreeOrTrees;
+        false -> V
+    end;
+subst_1({'*',Var}=V, Env) ->
+    case lists:keyfind(Var, 1, Env) of
+        {Var, TreeOrTrees} -> TreeOrTrees;
+        false -> V
+    end;
+subst_1(Leaf, _Env) ->
+    Leaf.
+
+
+-spec alpha(pattern_or_patterns(), [{id(), id()}]) -> template_or_templates().
+
+%% @doc Alpha converts a pattern (renames variables). Similar to tsubst/1,
+%% but only renames variables (including globs).
+%% @see tsubst/2
+
+alpha(Trees, Env) when is_list(Trees) ->
+    [alpha_1(template(T), Env) || T <- Trees];
+alpha(Tree, Env) ->
+    alpha_1(template(Tree), Env).
+
+alpha_1({template, Type, Attrs, Groups}, Env) ->
+    Gs1 = [lists:flatten([alpha_1(T, Env) || T <- G]) || G <- Groups],
+    {template, Type, Attrs, Gs1};
+alpha_1({Var}=V, Env) ->
+    case lists:keyfind(Var, 1, Env) of
+        {Var, NewVar} -> {NewVar};
+        false -> V
+    end;
+alpha_1({'*',Var}=V, Env) ->
+    case lists:keyfind(Var, 1, Env) of
+        {Var, NewVar} -> {'*',NewVar};
+        false -> V
+    end;
+alpha_1(Leaf, _Env) ->
+    Leaf.
+
+
+-spec match(pattern_or_patterns(), tree_or_trees()) ->
+                   {ok, env()} | error.
+
+%% @doc Match a pattern against a syntax tree (or patterns against syntax
+%% trees) returning an environment mapping variable names to subtrees; the
+%% environment is always sorted on keys. Note that multiple occurrences of
+%% metavariables in the pattern is not allowed, but is not checked.
+%%
+%% @see template/1
+%% @see switch/2
+
+match(Patterns, Trees) when is_list(Patterns), is_list(Trees) ->
+    try {ok, match_1(Patterns, Trees, [])}
+    catch
+        error -> error
+    end;
+match(Patterns, Tree) when is_list(Patterns) -> match(Patterns, [Tree]);
+match(Pattern, Trees) when is_list(Trees) -> match([Pattern], Trees);
+match(Pattern, Tree) ->
+    try {ok, match_template(template(Pattern), Tree, [])}
+    catch
+        error -> error
+    end.
+
+match_1([P|Ps], [T | Ts], Dict) ->
+    match_1(Ps, Ts, match_template(template(P), T, Dict));
+match_1([], [], Dict) ->
+    Dict;
+match_1(_, _, _Dict) ->
+    erlang:error(merl_match_arity).
+
+%% match a template against a syntax tree
+match_template({template, Type, _, Gs}, Tree, Dict) ->
+    case type(Tree) of
+        Type -> match_template_1(Gs, subtrees(Tree), Dict);
+        _ -> throw(error)  % type mismatch
+    end;
+match_template({Var}, _Tree, Dict)
+  when Var =:= '_' ; Var =:= 0 ->
+    Dict;  % anonymous variable
+match_template({Var}, Tree, Dict) ->
+    orddict:store(Var, Tree, Dict);
+match_template(Tree1, Tree2, Dict) ->
+    %% if Tree1 is not a template, Tree1 and Tree2 are both syntax trees
+    case compare_trees(Tree1, Tree2) of
+        true -> Dict;
+        false -> throw(error)  % different trees
+    end.
+
+match_template_1([G1 | Gs1], [G2 | Gs2], Dict) ->
+    match_template_2(G1, G2, match_template_1(Gs1, Gs2, Dict));
+match_template_1([], [], Dict) ->
+    Dict;
+match_template_1(_, _, _Dict) ->
+    throw(error).  % shape mismatch
+
+match_template_2([{Var} | Ts1], [_ | Ts2], Dict)
+  when Var =:= '_' ; Var =:= 0 ->
+    match_template_2(Ts1, Ts2, Dict);  % anonymous variable
+match_template_2([{Var} | Ts1], [Tree | Ts2], Dict) ->
+    match_template_2(Ts1, Ts2, orddict:store(Var, Tree, Dict));
+match_template_2([{'*',Var} | Ts1], Ts2, Dict) ->
+    match_glob(lists:reverse(Ts1), lists:reverse(Ts2), Var, Dict);
+match_template_2([T1 | Ts1], [T2 | Ts2], Dict) ->
+    match_template_2(Ts1, Ts2, match_template(T1, T2, Dict));
+match_template_2([], [], Dict) ->
+    Dict;
+match_template_2(_, _, _Dict) ->
+    throw(error).  % shape mismatch
+
+%% match the tails in reverse order; no further globs allowed
+match_glob([{'*',Var} | _], _, _, _) ->
+    fail("multiple glob variables in same match group: ~w", [Var]);
+match_glob([T1 | Ts1], [T2 | Ts2], Var, Dict) ->
+    match_glob(Ts1, Ts2, Var, match_template(T1, T2, Dict));
+match_glob([], _Group, Var, Dict) when Var =:= '_' ; Var =:= 0 ->
+    Dict;  % anonymous glob variable
+match_glob([], Group, Var, Dict) ->
+    orddict:store(Var, lists:reverse(Group), Dict);
+match_glob(_, _, _, _Dict) ->
+    throw(error).  % shape mismatch
+
+
+%% compare two syntax trees for equivalence
+compare_trees(T1, T2) ->
+    Type1 = type(T1),
+    case type(T2) of
+        Type1 ->
+            case subtrees(T1) of
+                [] ->
+                    case subtrees(T2) of
+                        [] -> compare_leaves(Type1, T1, T2);
+                        _Gs2 -> false  % shape mismatch
+                    end;
+                Gs1 ->
+                    case subtrees(T2) of
+                        [] -> false;  % shape mismatch
+                        Gs2 -> compare_trees_1(Gs1, Gs2)
+                    end
+            end;
+        _Type2 ->
+            false  % different tree types
+    end.
+
+compare_trees_1([G1 | Gs1], [G2 | Gs2]) ->
+    compare_trees_2(G1, G2) andalso compare_trees_1(Gs1, Gs2);
+compare_trees_1([], []) ->
+    true;
+compare_trees_1(_, _) ->
+    false.  % shape mismatch
+
+compare_trees_2([T1 | Ts1], [T2 | Ts2]) ->
+    compare_trees(T1, T2) andalso compare_trees_2(Ts1, Ts2);
+compare_trees_2([], []) ->
+    true;
+compare_trees_2(_, _) ->
+    false.  % shape mismatch
+
+compare_leaves(Type, T1, T2) ->
+    case Type of
+        atom ->
+            erl_syntax:atom_value(T1)
+                =:= erl_syntax:atom_value(T2);
+        char ->
+            erl_syntax:char_value(T1)
+                =:= erl_syntax:char_value(T2);
+        float ->
+            erl_syntax:float_value(T1)
+                =:= erl_syntax:float_value(T2);
+        integer ->
+            erl_syntax:integer_value(T1)
+                =:= erl_syntax:integer_value(T2);
+        string ->
+            erl_syntax:string_value(T1)
+                =:= erl_syntax:string_value(T2);
+        operator ->
+            erl_syntax:operator_name(T1)
+                =:= erl_syntax:operator_name(T2);
+        text ->
+            erl_syntax:text_string(T1)
+                =:= erl_syntax:text_string(T2);
+        variable ->
+            erl_syntax:variable_name(T1)
+                =:= erl_syntax:variable_name(T2);
+        _ ->
+            true  % trivially equal nodes
+    end.
+
+
+%% @doc Match against one or more clauses with patterns and optional guards.
+%%
+%% Note that clauses following a default action will be ignored.
+%%
+%% @see match/2
+
+-type switch_clause() ::
+          {pattern_or_patterns(), guarded_actions()}
+        | {pattern_or_patterns(), guard_test(), switch_action()}
+        | default_action().
+
+-type guarded_actions() :: guarded_action() | [guarded_action()].
+
+-type guarded_action() :: switch_action() | {guard_test(), switch_action()}.
+
+-type switch_action() :: fun( (env()) -> any() ).
+
+-type guard_test() :: fun( (env()) -> boolean() ).
+
+-type default_action() :: fun( () -> any() ).
+
+
+-spec switch(tree_or_trees(), [switch_clause()]) -> any().
+
+switch(Trees, [{Patterns, GuardedActions} | Cs]) when is_list(GuardedActions) ->
+    switch_1(Trees, Patterns, GuardedActions, Cs);
+switch(Trees, [{Patterns, GuardedAction} | Cs]) ->
+    switch_1(Trees, Patterns, [GuardedAction], Cs);
+switch(Trees, [{Patterns, Guard, Action} | Cs]) ->
+    switch_1(Trees, Patterns, [{Guard, Action}], Cs);
+switch(_Trees, [Default | _Cs]) when is_function(Default, 0) ->
+    Default();
+switch(_Trees, []) ->
+    erlang:error(merl_switch_clause);
+switch(_Tree, _) ->
+    erlang:error(merl_switch_badarg).
+
+switch_1(Trees, Patterns, GuardedActions, Cs) ->
+    case match(Patterns, Trees) of
+        {ok, Env} ->
+            switch_2(Env, GuardedActions, Trees, Cs);
+        error ->
+            switch(Trees, Cs)
+    end.
+
+switch_2(Env, [{Guard, Action} | Bs], Trees, Cs)
+  when is_function(Guard, 1), is_function(Action, 1) ->
+    case Guard(Env) of
+        true -> Action(Env);
+        false -> switch_2(Env, Bs, Trees, Cs)
+    end;
+switch_2(Env, [Action | _Bs], _Trees, _Cs) when is_function(Action, 1) ->
+    Action(Env);
+switch_2(_Env, [], Trees, Cs) ->
+    switch(Trees, Cs);
+switch_2(_Env, _, _Trees, _Cs) ->
+    erlang:error(merl_switch_badarg).
+
+
+%% ------------------------------------------------------------------------
+%% Internal utility functions
+
+-dialyzer({nowarn_function, fail/1}). % no local return
+
+fail(Text) ->
+    fail(Text, []).
+
+fail(Fs, As) ->
+    throw({error, lists:flatten(io_lib:format(Fs, As))}).
+
+flatten_text([L | _]=Lines) when is_list(L) ->
+    lists:foldr(fun(S, T) -> S ++ [$\n | T] end, "", Lines);
+flatten_text([B | _]=Lines) when is_binary(B) ->
+    lists:foldr(fun(S, T) -> binary_to_list(S) ++ [$\n | T] end, "", Lines);
+flatten_text(Text) when is_binary(Text) ->
+    binary_to_list(Text);
+flatten_text(Text) ->
+    Text.
+
+-spec metavar(tree()) -> {string()} | false.
+
+%% Check if a syntax tree represents a metavariable. If not, 'false' is
+%% returned; otherwise, this returns a 1-tuple with a string containing the
+%% variable name including lift/glob prefixes but without any leading
+%% metavariable prefix, and instead prefixed with "v" for a variable or "i"
+%% for an integer.
+%%
+%% Metavariables are atoms starting with @, variables starting with _@,
+%% strings starting with "'@, or integers starting with 909. Following the
+%% prefix, one or more _ or 0 characters (unless it's the last character in
+%% the name) may be used to indicate "lifting" of the variable one or more
+%% levels , and after that, a @ or 9 character indicates a glob metavariable
+%% rather than a normal metavariable. If the name after the prefix is _ or
+%% 0, the variable is treated as an anonymous catch-all pattern in matches.
+
+metavar(Tree) ->
+    case type(Tree) of
+        atom ->
+            case erl_syntax:atom_name(Tree) of
+                "@" ++ Cs when Cs =/= [] -> {"v"++Cs};
+                _ -> false
+            end;
+        variable ->
+            case erl_syntax:variable_literal(Tree) of
+                "_@" ++ Cs when Cs =/= [] -> {"v"++Cs};
+                _ -> false
+            end;
+        integer ->
+            case erl_syntax:integer_value(Tree) of
+                N when N >= 9090 ->
+                    case integer_to_list(N) of
+                        "909" ++ Cs -> {"n"++Cs};
+                        _ -> false
+                    end;
+                _ -> false
+            end;
+        string ->
+            case erl_syntax:string_value(Tree) of
+                "'@" ++ Cs -> {"v"++Cs};
+                _ -> false
+            end;
+        _ ->
+            false
+    end.
+
+%% wrappers around erl_syntax functions to provide more uniform shape of
+%% generic subtrees (maybe this can be fixed in syntax_tools one day)
+
+type(T) ->
+    case erl_syntax:type(T) of
+        nil  -> list;
+        Type -> Type
+    end.
+
+subtrees(T) ->
+    case erl_syntax:type(T) of
+        tuple ->
+            [erl_syntax:tuple_elements(T)];  %% don't treat {} as a leaf
+        nil ->
+            [[], []];  %% don't treat [] as a leaf, but as a list
+        list ->
+            case erl_syntax:list_suffix(T) of
+                none ->
+                    [erl_syntax:list_prefix(T), []];
+                S ->
+                    [erl_syntax:list_prefix(T), [S]]
+            end;
+        binary_field ->
+            [[erl_syntax:binary_field_body(T)],
+             erl_syntax:binary_field_types(T)];
+        clause ->
+            case erl_syntax:clause_guard(T) of
+                none ->
+                    [erl_syntax:clause_patterns(T), [],
+                     erl_syntax:clause_body(T)];
+                G ->
+                    [erl_syntax:clause_patterns(T), [G],
+                     erl_syntax:clause_body(T)]
+            end;
+        receive_expr ->
+            case erl_syntax:receive_expr_timeout(T) of
+                none ->
+                    [erl_syntax:receive_expr_clauses(T), [], []];
+                E ->
+                    [erl_syntax:receive_expr_clauses(T), [E],
+                     erl_syntax:receive_expr_action(T)]
+            end;
+        record_access ->
+            case erl_syntax:record_access_type(T) of
+                none ->
+                    [[erl_syntax:record_access_argument(T)], [],
+                     [erl_syntax:record_access_field(T)]];
+                R ->
+                    [[erl_syntax:record_access_argument(T)], [R],
+                     [erl_syntax:record_access_field(T)]]
+            end;
+        record_expr ->
+            case erl_syntax:record_expr_argument(T) of
+                none ->
+                    [[], [erl_syntax:record_expr_type(T)],
+                     erl_syntax:record_expr_fields(T)];
+                V ->
+                    [[V], [erl_syntax:record_expr_type(T)],
+                     erl_syntax:record_expr_fields(T)]
+            end;
+        record_field ->
+            case erl_syntax:record_field_value(T) of
+                none ->
+                    [[erl_syntax:record_field_name(T)], []];
+                V ->
+                    [[erl_syntax:record_field_name(T)], [V]]
+            end;
+        _ ->
+            erl_syntax:subtrees(T)
+    end.
+
+make_tree(list, [P, []]) -> erl_syntax:list(P);
+make_tree(list, [P, [S]]) -> erl_syntax:list(P, S);
+make_tree(tuple, [E]) -> erl_syntax:tuple(E);
+make_tree(binary_field, [[B], Ts]) -> erl_syntax:binary_field(B, Ts);
+make_tree(clause, [P, [], B]) -> erl_syntax:clause(P, none, B);
+make_tree(clause, [P, [G], B]) -> erl_syntax:clause(P, G, B);
+make_tree(receive_expr, [C, [], _A]) -> erl_syntax:receive_expr(C);
+make_tree(receive_expr, [C, [E], A]) -> erl_syntax:receive_expr(C, E, A);
+make_tree(record_access, [[E], [], [F]]) -> erl_syntax:record_access(E, F);
+make_tree(record_access, [[E], [T], [F]]) -> erl_syntax:record_access(E, T, F);
+make_tree(record_expr, [[], [T], F]) -> erl_syntax:record_expr(T, F);
+make_tree(record_expr, [[E], [T], F]) -> erl_syntax:record_expr(E, T, F);
+make_tree(record_field, [[N], []]) -> erl_syntax:record_field(N);
+make_tree(record_field, [[N], [E]]) -> erl_syntax:record_field(N, E);
+make_tree(Type, Groups) ->
+    erl_syntax:make_tree(Type, Groups).
+
+merge_comments(_StartLine, [], [T]) -> T;
+merge_comments(_StartLine, [], Ts) -> Ts;
+merge_comments(StartLine, Comments, Ts) ->
+    merge_comments(StartLine, Comments, Ts, []).
+
+merge_comments(_StartLine, [], [], [T]) -> T;
+merge_comments(_StartLine, [], [T], []) -> T;
+merge_comments(_StartLine, [], Ts, Acc) ->
+    lists:reverse(Acc, Ts);
+merge_comments(StartLine, Cs, [], Acc) ->
+    merge_comments(StartLine, [], [],
+                   [erl_syntax:set_pos(
+                      erl_syntax:comment(Indent, Text),
+                      StartLine + Line - 1)
+                    || {Line, _, Indent, Text} <- Cs] ++ Acc);
+merge_comments(StartLine, [C|Cs], [T|Ts], Acc) ->
+    {Line, _Col, Indent, Text} = C,
+    CommentLine = StartLine + Line - 1,
+    case erl_syntax:get_pos(T) of
+        Pos when Pos < CommentLine ->
+            %% TODO: traverse sub-tree rather than only the top level nodes
+            merge_comments(StartLine, [C|Cs], Ts, [T|Acc]);
+        CommentLine ->
+            Tc = erl_syntax:add_postcomments(
+                   [erl_syntax:comment(Indent, Text)], T),
+            merge_comments(StartLine, Cs, [Tc|Ts], Acc);
+        _ ->
+            Tc = erl_syntax:add_precomments(
+                   [erl_syntax:comment(Indent, Text)], T),
+            merge_comments(StartLine, Cs, [Tc|Ts], Acc)
+    end.