static <A, B> L<Pair<A, B>> zipTwoListsToPairs(L<A> l1, L<B> l2) {
  int n = min(l(l1), l(l2));
  LPair<A, B> out = emptyList(n);
  for i to n: addPair(out, l1.get(i), l2.get(i));
  ret out;
}