anchor.diff:  attach(Interface) ->
       gen_event:add_handler(enet_iface:event_manager(Interface),
                             ?MODULE, []).
  +
  +change_format(Interface, Format) ->
  +    gen_event:call(enet_iface:event_manager(Interface),
  +                   ?MODULE, {change_format, Format}).
   
   %%%===================================================================
   %%% gen_event callbacks
  
positive.code: 
  -include("types.hrl").
  
  -record(state, {print=true}).
  
  %%%===================================================================
  %%% gen_event callbacks
  
negative.code: %%                   {remove_handler, Reply}
  %% @end
  %%--------------------------------------------------------------------
  handle_call({change_format, Format}, State) ->
      {ok, ok, State#state{print=Format}}.
  
  %%--------------------------------------------------------------------
  %% @private
  