anchor.diff:    /// When null, no limit is applied. Set this to 1 if you want to only allow
     /// the drag source to have one item dragged at a time. Set this to 0 if you
     /// want to prevent the draggable from actually being dragged.
  +  ///
  +  /// If you set this property to 1, consider supplying an "empty" widget for
  +  /// [childWhenDragging] to create the illusion of actually moving [child].
     final int maxSimultaneousDrags;
   
     /// Called when the draggable starts being dragged.
  
positive.code: /// When a draggable widget recognizes the start of a drag gesture, it displays
  /// a [feedback] widget that tracks the user's finger across the screen. If the
  /// user lifts their finger while on top of a [DragTarget], that target is given
  /// the opportunity to accept the [data] carried by the draggble.
  ///
  /// See also:
  ///
  
negative.code:   final T data;
  
    /// The widget below this widget in the tree.
    ///
    /// This widget displays [child] when zero drags are under way. If
    /// [childWhenDragging] is non-null, this widget instead displays
    /// [childWhenDragging] when one or more drags are underway. Otherwise, this
    /// widget always displays [child].
    ///
    /// The [feedback] widget is shown under the pointer when a drag is under way.
    ///
    /// To limit the number of simultaneous drags on multitouch devices, see
    /// [maxSimultaneousDrags].
    final Widget child;
  
    /// The widget to display instead of [child] when one or more drags are under way.
    ///
    /// If this is null, then this widget will always display [child] (and so the
    /// drag source representation will not change while a drag is under
    /// way).
    ///
    /// The [feedback] widget is shown under the pointer when a drag is under way.
    ///
    /// To limit the number of simultaneous drags on multitouch devices, see
    /// [maxSimultaneousDrags].
    final Widget childWhenDragging;
  
    /// The widget to show under the pointer when a drag is under way.
    ///
    /// See [child] and [childWhenDragging] for information about what is shown
    /// at the location of the [Draggable] itself when a drag is under way.
    final Widget feedback;
  
    /// The feedbackOffset can be used to set the hit test target point for the
  