anchor.diff:  
       test('it basically works', () {
   
  -      // This is a very crude test, not covering much of the code base
  -      // Some ideas of tests :
  -      // - backward events
  -      // - meaningful exceptions :
  -      //   - myClass with null properties
  -      //   - myClass with non-numeric properties
  +      var myClass = new MyClass();
   
  +      // Register the accessor
         Tween.registerAccessor(MyClass, new MyAccessor());
   
         // The following are expected to be called exactly once
  
positive.code: 
    });
  
  }
negative.code:       myCallback.onEvent = (type, tween) {
          switch(type) {
            case TweenCallback.BEGIN:
              expectOnBegin(tween);
              break;
            case TweenCallback.COMPLETE:
              expectOnComplete(tween);
              break;
            case TweenCallback.START:
              expectOnStart(tween);
              break;
            case TweenCallback.END:
              expectOnEnd(tween);
              break;
            default:
              print('DEFAULT CALLBACK CAUGHT ; type = ' + type.toString());
          }
        };
  
        Tween.to(myClass, MyAccessor.XY, 1.0)
          ..targetValues = [20, 30]
          ..easing = Elastic.INOUT
  