anchor.diff:    
     /// The function to be called when the mouse is dragged (back) into the original element
     DragCallback dragOver;
  +  
  +  /// The function to call to determine if the drag should be allowed to start on mouse down
  +  AllowDragStart dragConditions;
     
     // references to the local mouse event handlers
     var _mouseDown;
  
positive.code:   }
    
    void _mouseDownHandler(MouseEvent event) {
      //print("mouse down, enabled: $enabled");
      // TODO if(!_dragging)?
      
  
negative.code:   
    /// Add a target to the set
    void addTarget(Element element) {
      print("adding target");
      // save size to compare with size after
      int oldSize = _targets.length;
      // add element
  