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;
}