// Finds out the index of a sublist in the original list
// Works with nested (grand-parent) sublists.
// Does not work with lists not made with subList() (returns -1)

static <A> int magicIndexOfSubList(L<A> list, L<A> sublist) {
  Integer o1 = cast getOpt(list, "offset");
  if (o1 == null) ret -1;
  int o2 = (Int) get(sublist, "offset");
  ret o2-(o1 != null ? o1 : 0);
}