static <A> SplittableSteppable splittableSteppable_forEach(final Iterable<A> items, final VF1<A> f) {
  ret new SplittableSteppable {
    Iterator<A> it = iterator(items);
    public bool step() { // return false if done
      if (!it.hasNext()) false;
      callF(f, it.next());
      true;
    }
    
    // invalidates original steppable
    public L<Steppable> split(int n) {
      final Lock lock = lock();
      ret rep((Steppable) new Steppable {
        public bool step() {
          A a;
          {
            lock lock;
            if (!it.hasNext()) false;
            a = it.next();
          }
          callF(f, a);
          true;
        }
      }, n);
    }
  };
}