anchor.diff:        appendOuput('---');
       }
     }
  -  
  +
     void listenForEventB() {
       if (subscription != null) {
         appendOuput('Already listening for an event.');
  
positive.code:       appendOuput('---');
      }
    }
    
    void pause() {
      if (subscription != null) {
        subscription.pause();
  
negative.code: void main() {
    // Init logging.
    initLogging();
  
    // Log all events.
    eventBus.on().listen((event) => _log.finest('event fired:  ${event.runtimeType}'));
  
  
    // Initialize the listener boxes.
    Listener listener1 = new Listener(querySelector('#listener-1'));
    Listener listener2 = new Listener(querySelector('#listener-2'));
  
    // Init Event fields.
    LabelElement fireLabelA = querySelector('#fire-label-a');
    LabelElement fireLabelB = querySelector('#fire-label-b');
    ButtonElement fireButtonA = querySelector("#fire-button-a");
    ButtonElement fireButtonB = querySelector("#fire-button-b");
  
    fireButtonA.onClick.listen((_) {
      // -------------------------------------------------
      // Fire Event A
  