static persistable class ListWalker<A> implements Iterator<A> {
  L<A> list;
  int index;
  
  *(L<A> *list) {}
  
  public A next() {
    if (index < l(list))
      ret list.get(index++);
    null;
  }
  
  public bool hasNext() {
    ret index < l(list);
  }
}