anchor.diff:    List<Element> _elements() {
       return [
         this._currentDrill.questionDiv(),
  -      this.button()
  +      new NextStateButton().element()
         ];
     }
   
  
positive.code:     return [
        this._currentDrill.questionDiv(),
        this._currentDrill.answerDiv(),
        this.button()
      ];
    }
  
  
negative.code:   }
  }
  
  // on the global object so that the NextStateButton can call it
  App app;
  
  void main() {
  