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diff --git a/src/cowboy_sub_protocol.erl b/src/cowboy_sub_protocol.erl
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+%% Copyright (c) 2013, James Fish <[email protected]>
+%%
+%% Permission to use, copy, modify, and/or distribute this software for any
+%% purpose with or without fee is hereby granted, provided that the above
+%% copyright notice and this permission notice appear in all copies.
+%%
+%% THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+%% WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+%% MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+%% ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+%% WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+%% ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+%% OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+
+%% @doc Behaviour for sub protocols.
+%%
+%% Only one function needs to be implemented, <em>upgrade/4</em>.
+%% It receives the Req, the environment, the handler that the request has been
+%% routed to and the handler's options. It acts exactly the same as a
+%% middleware, so returns the same values a middleware's execute/2.
+%%
+%% Once the sub protocol has processed the request it should add the result
+%% to the environment. This is done by adding the tuple {result, Value} to the
+%% environment list. To continue handling requests on the current connection the
+%% Value should be the atom ok. Any other value will prevent the processing of
+%% subsequent requests.
+%%
+%% <em>upgrade/4</em> will be called when a handler's init/3 returns
+%% {upgrade, protocol, Module}, where Module is the module of the sub protocol.
+-module(cowboy_sub_protocol).
+
+-callback upgrade(Req, Env, module(), any())
+	-> {ok, Req, Env}
+	| {suspend, module(), atom(), any()}
+	| {halt, Req}
+	| {error, cowboy_http:status(), Req}
+	when Req::cowboy_req:req(), Env::cowboy_middleware:env().