static JPanel smartAdd(JPanel panel, L parts) {
  for (O o : parts) {
    Component c;
    if (o == null)
      c = new JPanel;
    else if (o instanceof S)
      c = jlabel((S) o);
    else
      c = wrap(o);
    panel.add(c);
  }
  ret panel;
}

static JPanel smartAdd(JPanel panel, O... parts) {
  ret smartAdd(panel, asList(parts));
}