private static void handleExpansion(TreeExpansionEvent event, boolean expanded) {
        final TreePath path = event.getPath();
        final Object component = path != null ? path.getLastPathComponent() : null;
        if (component instanceof XValueGroupNodeImpl) {
          ((XValueGroupNodeImpl)component).onExpansion(expanded);
        }
      }
private static void handleExpansion(TreeExpansionEvent event, boolean expanded)