static <A, B> L<Pair<A, B>> combineZigZag(L<A> l1, L<B> l2) {
  int lA = l(l1), lB = l(l2), maxSum = lA+lB;
  assertEquals("lists length", lA, lB);
  L<Pair<A, B>> out = new L;
  for sum to maxSum: {
    int maxA = min(lA, sum);
    for iA to maxA:
      out.add(pair(l1.get(iA), l2.get(sum-1-iA)));
  }
  ret out;
}