// f: voidfunc(int)
svoid onRadioButtonChange(final ButtonGroup group, fO f) {
  final Var<Int> idx = new Var(selectedButtonIdx(group));
  Runnable r = r {
    int idx2 = selectedButtonIdx(group);
    if (idx2 >= 0 && idx2 != idx.get()) {
      idx.set(idx2);
      pcallF(f, idx2);
    }
  };
  for (AbstractButton b : buttonsInGroup(group))
    onChange(b, r);
}

svoid onRadioButtonChange(ButtonGroup group, IVF1<Int> f) {
  onRadioButtonChange(group, (O) f);
}