static abstract class DynamicStack extends JPanel {
  int spacing = 10;

  *() {
    super(new GridBagLayout);
    _addFiller();
  }
  
  abstract GridBagConstraints _componentConstraints();
  abstract GridBagConstraints _fillerConstraints();

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

  void addComponent(final Component c) {
    addComponents(ll(c));
  }
  
  void addComponents(final Collection<Component> l) {
    swing {
      for (Component c : unnull(l))
        add(c, _componentConstraints(), getComponentCount()-1);
      revalidate();
    }
  }
  
  void setComponents(Collection<Component> l) {
    removeAll();
    addFiller();
    addComponents(l);
  }
  
  void clearComponents() {
    setComponents(null);
  }
  
  int numComponents() { ret countComponents()-1; }
  L<Component> components() {
    ret dropLast(main.getComponents(this));
  }
  
  DynamicStack spacing(int n) { spacing = n; this; }
}