static <A> int indexOfDifference(Iterable<A> i1, Iterable<A> i2) {
  var it1 = iterator(i1);
  var it2 = iterator(i2);
  int i = 0;
  while ping (true) {
    bool a = it1.hasNext(), b = it2.hasNext();
    if (a != b)
      ret i;
    if (!a)
      ret -1;
    A el1 = it1.next();
    A el2 = it2.next();
    if (!eq(el1, el2))
      ret i;
    ++i;
  }
}