static <A, B> Map<A, B> zipTwoListsToMap_strict(L<A> l1, L<B> l2) {
  int n = l(l1);
  if (n != l(l2)) null;
  new Map map;
  for i to n:
    if (!strictPut(map, l1.get(i), l2.get(i)))
      null;
  ret map;
}