static <A, B> L<Pair<A, B>> combineZigZag(L<A> l1, L<A> l2) {
  int lA = l(l1), lB = l(l2), maxSum = lA+lB-1;
  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-iA)));
  }
  ret out;
}