anchor.diff:      
       window.requestAnimationFrame(callbackStart);
     });
  +  
  +  document.onKeyPress.where((e) => e.keyCode == 82).listen((e) {
  +    var boxes = new List.from(boxManager.boxes);
  +    boxes.forEach((box) {
  +      if (mainBox != box) {
  +        boxManager.remove(box);
  +      }
  +    });
  +  });
   }
positive.code: import "package:dnd/dnd.dart";
  
  void main() {
    var box = boxManager.create(128, 128);
    box.appendTo(document.body);
    box.setColor("blue", background: true);
    box.enableDragging(new Dropzone(document.body));
  
    boxManager.moveApart(20);
    document.onKeyPress.where((it) => it.keyCode == 67).listen((e) {
      var newBox = boxManager.clone(box);
      var e = newBox.element;
      
      int i = 0;
  
negative.code:     }
      
      void callbackStart(_) {
        if (i == 7) {
          window.requestAnimationFrame(callbackEnd);
          return;
        }
  