public override void ReplaceChild(ILayoutElement oldElement, ILayoutElement newElement)
 {
     Debug.Assert(Equals(oldElement, RootPanel) && oldElement != null);
     RootPanel = newElement as LayoutAnchorablePaneGroup;
 }
 public override void RemoveChild(ILayoutElement element)
 {
     Debug.Assert(Equals(element, RootPanel) && element != null);
     RootPanel = null;
 }