static <A, B> L<Pair<A, B>> combineZigZag(L<A> l1, L<B> l2) {
  int lA = l(l1), lB = l(l2), maxSum = max(lA, lB);
  L<Pair<A, B>> out = new L;
  
  int ndiags = lA + lB - 1;
  for diag to ndiags: {
    int row_stop = max(0, diag - lA + 1);
    int row_start = min(diag, lB - 1);
    for (int row = row_start; row >= row_stop; row--) {
      // on a given diagonal row + col = constant "diag"
      // diag labels the diagonal number
      int col = diag - row;
      out.add(pair(l1.get(col), l2.get(row)));
    }
  }

  ret out;
}