anchor.diff:  add_sup_callback(Fn) when is_function(Fn) ->
       gen_event:add_sup_handler(?MODULE, {?MODULE, make_ref()}, [Fn]).
   
  +%% @todo Change back to non.
   update(LocalState) ->
  -    gen_event:notify(?MODULE, {update, LocalState}).
  +    gen_event:sync_notify(?MODULE, {update, LocalState}).
   
   %% ===================================================================
   %% gen_event callbacks
  
positive.code: 
  init([Fn]) ->
      {ok, LocalState} = ?PEER_SERVICE_MANAGER:get_local_state(),
      Fn(LocalState),
      {ok, #state {callback = Fn}}.
  
  handle_event({update, LocalState}, State) ->
      (State#state.callback)(LocalState),
      {ok, State}.
  
  handle_call(_Request, State) ->
      {ok, ok, State}.
  
  handle_info(_Info, State) ->
      {ok, State}.
  
  terminate(_Reason, _State) ->
  
negative.code:          terminate/2,
           code_change/3]).
  
  -record(state, {callback}).
  
  %% ===================================================================
  %% API functions
  