static persistable class ImmutableListWalker<A> implements Iterator<A> {
  L<A> list;
  int index;
  
  *(L<A> *list) {}
  *(L<A> *list, int *index) {}
  
  public A has() { ret index >= 0 && index < l(list); }
  
  public A get() {
    ret _get(list, index);
  }
  
  public ImmutableListWalker<A> next() {
    ret new ImmutableListWalker(list, index+1);
  }
}