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+cowboy_sub_protocol
+===================
+
+The `cowboy_sub_protocol` behaviour defines the interface used
+by modules that implement a protocol on top of HTTP.
+
+Types
+-----
+
+None.
+
+Callbacks
+---------
+
+### upgrade(Req, Env, Handler, Opts)
+	-> {ok, Req, Env}
+	| {suspend, Module, Function, Args}
+	| {halt, Req}
+	| {error, StatusCode, Req}
+
+> Types:
+>  *  Req = cowboy_req:req()
+>  *  Env = env()
+>  *  Handler = module()
+>  *  Opts = any()
+>  *  Module = module()
+>  *  Function = atom()
+>  *  Args = [any()]
+>  *  StatusCode = cowboy:http_status()
+>
+> Upgrade the protocol.
+>
+> Please refer to the `cowboy_middleware` manual for a
+> description of the return values.