anchor.diff:    }
   
     /// Specify the child's origin relative to this origin.
  +  ///
  +  /// Call this from your [performLayout] function to position each
  +  /// child. If you do not call this for a child, its position will
  +  /// remain unchanged. Children initially have their position set to
  +  /// (0,0), i.e. the top left of the [RenderCustomMultiChildLayoutBox].
     void positionChild(Object childId, Offset offset) {
       final RenderBox child = _idToChild[childId];
       assert(() {
  
positive.code: 
  // For OneChildLayoutDelegate and RenderCustomOneChildLayoutBox, see shifted_box.dart
  
  class MultiChildLayoutParentData extends ContainerBoxParentDataMixin<RenderBox> {
    /// An object representing the identity of this child.
    Object id;
  
negative.code:     }
    }
  
    /// Override this method to return the size of this object given the
    /// incoming constraints. The size cannot reflect the instrinsic
    /// sizes of the children. If this layout has a fixed width or
    /// height the returned size can reflect that; the size will be
    /// constrained to the given constraints.
    ///
    /// By default, attempts to size the box to the biggest size
    /// possible given the constraints.
    Size getSize(BoxConstraints constraints) => constraints.biggest;
  
    /// Override this method to lay out and position all children given
    /// this widget's size and the specified constraints. This method
    /// must call [layoutChild] for each child. It should also specify
    /// the final position of each child with [positionChild].
    void performLayout(Size size, BoxConstraints constraints);
  
    /// Override this method to return true when the children need to be
    /// laid out. This should compare the fields of the current delegate
    /// and the given oldDelegate and return true if the fields are such
    /// that the layout would be different.
    bool shouldRelayout(MultiChildLayoutDelegate oldDelegate);
  
    /// Override this method to include additional information in the
    /// debugging data printed by [debugDumpRenderTree] and friends.
    ///
    /// By default, returns the [runtimeType] of the class.
    String toString() => '$runtimeType';
  }
  
  