anchor.diff:      counter= prettyPrintTime(25*60);
       currentState = ClockState.pomodoro;
       currentDuration = startPomodoroAt;
  -    times = new List();
  +    periods = new List();
     }
     
     void start() {
  
positive.code: </p>
  <p>History:</p>
  <ul>
      <li *ng-for="#time of times">
          {{time}}
      </li>
  </ul>
  ''', directives: const [NgFor, NgIf])
  
negative.code:   void addToHistory(){
      switch(currentState){
        case ClockState.pomodoro:
          var now = new DateTime.now();
          var start = now.subtract(new Duration(seconds: startRestAt));
          var end = now;
          periods.add(new Period(start, end,"rest"));
          break;
        case ClockState.rest:
          var now = new DateTime.now();
          var start = now.subtract(new Duration(seconds: startPomodoroAt));
          var end = now;
          periods.add(new Period(start, end,"pomodoro"));
          break;
      }
    }
  