// Don't call add/remove directly - use addComponent/removeComponent
static abstract class DynamicStack extends JPanel {
  settable int spacing = 10;
  bool useFiller;

  *() { this(true); }
  
  *(bool useFiller) {
    super(new GridBagLayout);
    this.useFiller = useFiller;
    _addFiller();
  }
  
  abstract GridBagConstraints _componentConstraints();
  abstract GridBagConstraints _fillerConstraints();

  void _addFiller {
    if (useFiller) add(jrigid(), _fillerConstraints());
  }

  void addComponent(final Component c) {
    addComponents(ll(c));
  }
  
  void addComponents(Cl<? extends Component> l) swing {
    fOr (Component c : l)
      if (c.getParent() != this)
        add(c, _componentConstraints(), getComponentCount()-(useFiller ? 1 : 0));
    revalidate();
  }
  
  void setComponents(Cl<? extends Component> l) {
    removeAll();
    _addFiller();
    addComponents(l);
  }
  
  void clearComponents() {
    setComponents(null);
  }
  
  void removeComponent(Component c) swing {
    if (c != null) {
      remove(c);
      revalidate();
    }
  }
  
  int numComponents() { ret countComponents()-1; }
  L<Component> components() {
    ret dropLast(main getComponents(this));
  }
  
  bool isEmpty() { ret numComponents() == 0; }
}