anchor.diff:    List<Element> _elements() {
       return [
         this._currentDrill.questionDiv(),
  -      this.button()
  +      new NextStateButton().element()
         ];
     }
   
  
positive.code:   }
  }
  
  App app;
  
  void main() {
  
negative.code:   State nextState() {
      return new StartState();
    }
  }
  
  /// A button that tells the app to move to the next state
  class NextStateButton {
    Element _button;
    NextStateButton() {
  	this._button = new ButtonElement();
      this._button.text = "click";
      this._button.onClick.listen((e) => app.displayNextState());
    }
  
    Element element() {
      return this._button;
    }
  
  }
  
  /// Represents the visual app in front of the user.
  