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); }
  ListAndIndex<A> minus(int ofs) { ret ListAndIndex(list, idx-ofs); }
  
  L<A> list() { ret list; }
  int index aka idx() { ret idx; }
  
  ListAndIndex<A> mapIdx(IF1_IntToInt f) {
    ret ListAndIndex<A>(list, f.get(idx));
  }
}