static <A extends JButton> A treeDependButton(final JTree tree, final A b) {
  if (b != null) swing {
    tree.addTreeSelectionListener(new TreeSelectionListener {
      public void valueChanged(TreeSelectionEvent e) {
        b.setEnabled(getSelectedTreeNode(tree) != null);
      }
    });
    b.setEnabled(getSelectedTreeNode(tree) != null);
  }
  ret b;
}