static <A, B> L<Pair<A, B>> zipTwoListsToPairs_ifSameLength(L<A> l1, L<B> l2) {
  int n = l(l1);
  if (n != l(l2)) null;
  new L<Pair<A, B>> out;
  for i to n: addPair(out, l1.get(i), l2.get(i));
  ret out;
}