srecord ListAndIndex<A>(L<A> list, int idx) {
  bool atEnd() { ret idx >= l(list); }
  
  A get() { ret _get(list, idx); }
  int size() { ret l(list); }
  
  toString {
    ret subList(list, 0, idx) + ", then " + subList(list, idx);
  }
  
  ListAndIndex<A> plus(int ofs) { ret ListAndIndex(list, idx+ofs); }
  
  int index() { ret idx; }
}