sclass SingleComponentPanel extends JPanel {
  *() { super(new BorderLayout); }

  *(Component component) {
    this();
    if (component != null) setComponent(component);
  }

  void setComponent(Component component) {
    removeAll();
    if (component != null)
      add(BorderLayout.CENTER, wrap(component));
    main.revalidate(this);
  }
  
  void noComponent() { setComponent(null); }
}