for (int c0 = 0; c0 <= M; c0 += 1) {
  for (int c1 = 0; c1 <= min(N, c0); c1 += 1) {
    S1(c0, c1);
    S2(c0, c1);
  }
  for (int c1 = N + 1; c1 <= c0; c1 += 1)
    S1(c0, c1);
}