// the walker is immutable, meaning the only way to
// advance the pointer is to make a new walker object
static persistable class ImmutableListWalker<A> {
  L<A> list;
  int index;
  
  *(L<A> *list) {}
  *(L<A> *list, int *index) {}
  
  bool has() { ret index >= 0 && index < l(list); }
  
  A get() {
    ret _get(list, index);
  }
  
  ImmutableListWalker<A> next() {
    ret new ImmutableListWalker(list, index+1);
  }
  
  bool atEnd() {
    ret index >= l(list);
  }
}

Began life as a copy of #1030668