anchor.diff:  
   // TODO: 'alpha' does not compose well. If a parent is 50% and the child is
   //       50% the render should be at %25, right? :-/
  +// TODO: support clipping - https://github.com/kevmoo/bot.dart/issues/15
   
   abstract class Thing extends AttachableObject {
     final List<AffineTransform> _transforms = new List<AffineTransform>();
  
positive.code:   num _width, _height, _alpha = 1;
    bool _cacheEnabled = false;
    Size _lastDrawSize;
    bool clip = false;
    ThingParent _parent;
  
    Thing(this._width, this._height);
  
negative.code:     drawOverride(ctx);
    }
  
    void _invalidateParent(){
      assert(this._parent != null);
      _invalidatedEventHandle.fireEvent(EventArgs.empty);
  