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