static JComboBox onSelectedItem(final JComboBox cb, final VF1<S> f) {
  addActionListener(cb, r {
    pcallF(f, selectedItem(cb))
  });
  ret cb;
}

static JComboBox onSelectedItem(final JComboBox cb, IVF1<S> f) {
  addActionListener(cb, r {
    pcallF(f, selectedItem(cb))
  });
  ret cb;
}