static <A> L<Triple<A, A, Int>> differencesElementByElement(L<A> a, L<A> b) {
  L<Triple<A, A, Int>> l = new L;
  if (l(a) != l(b)) ret l;
  int n = l(a);
  for i to n: {
    A aa = a.get(i);
    A bb = b.get(i);
    if (neq(aa, bb))
      l.add(triple(aa, bb, i));
  }
  ret l;
}