static <A> Steppable steppable_forEach(Iterable<A> items, VF1<A> f) {
  ret steppable_forEach(items, (O) f);
}

static <A> Steppable steppable_forEach(Iterable<A> items, IVF1<A> f) {
  ret steppable_forEach(items, (O) f);
}

static <A> Steppable steppable_forEach(Iterable<A> items, O f) {
  ret new Steppable {
    Iterator<A> it = iterator(items);
    public bool step() { // return false if done
      if (!it.hasNext()) false;
      callF(f, it.next());
      true;
    }
  };
}