srecord noeq ForEach_vstack<A>(Iterable<A> l) extends VStackComputableWithStep {
  Iterator<A> it;
  
  void step {
    if (step == 0) {
      if (l == null) ret with stack.ret();
      it = iterator(l);
      ++step;
    }
    
    if (!it.hasNext())
      ret with stack.ret();
    
    ret with stack.ret(it.next());
  }
}