anchor.diff:  handle_call(get_open_reqs, _From, State) ->
       {reply, {ok, State#state.open_reqs}, State};
   
  +handle_call({set_callback, Callback, CallbackArgs}, _From, State) ->
  +    ok = Callback:handle_event(elli_reconfigure, [], CallbackArgs),
  +    {reply, ok, State#state{callback = {Callback, CallbackArgs}}};
  +
   handle_call(stop, _From, State) ->
       {stop, normal, ok, State}.
   
  
positive.code:          , get_acceptors/1
           , get_open_reqs/1
           , get_open_reqs/2
          ]).
  
  %% gen_server callbacks
  
negative.code: 
  get_open_reqs(S, Timeout) ->
      gen_server:call(S, get_open_reqs, Timeout).
  
  set_callback(S, Callback, CallbackArgs) ->
      valid_callback(Callback) orelse throw(invalid_callback),
      gen_server:call(S, {set_callback, Callback, CallbackArgs}).
  
  stop(S) ->
      gen_server:call(S, stop).
  