anchor.diff:              {stop, {error,[{reason, Reason}, {stacktrace, erlang:get_stacktrace()}]}}
       end.
   
  +handle_call(shutdown, _From, State) ->
  +    {stop, shutdown, ok, State};
   handle_call(_Request, _From, State) ->
       Reply = ok,
       {reply, Reply, State}.
  
positive.code: -export([start_link/4,
           start/4,
           notify/2,
           behaviour_info/1]).
  
  %% gen_server callbacks
  
negative.code:            Args,
             Opts) ->
      gen_server:start(ServerName, ?MODULE, {ElipsBehavior, Args}, Opts).
  
  shutdown(ServerRef) ->
      ok=gen_server:call(ServerRef, shutdown).
  
  
  notify(ServerRef, Event) ->
  