anchor.diff:  
   -export([new_session/0]).
   -export([get_session/1]).
  --export([session_closing/1]).
  +-export([session_wants_to_close/1]).
   %% gen_server.
   -export([init/1]).
   -export([handle_call/3]).
  
positive.code:     Pid = start_session(ID),
      ok = set_session_for_id(ID,Pid),
      {reply, {ok, Pid}, State};
  handle_call(_Request, _From, State) ->
  	{reply, ignored, State}.
  
  
negative.code: get_session(ID) ->
      lookup_or_create_session_pid(ID).
  
  -spec session_wants_to_close(ID :: binary()) -> ok.
  session_wants_to_close(ID) ->
      gen_server:call(?MODULE,{delete_session,ID}).
  
  %% gen_server.
  
  