for (int c1 = 1; c1 <= M; c1 += 1)
  S2(c1);
for (int c0 = 2; c0 <= M; c0 += 1) {
  for (int c2 = 1; c2 < c0; c2 += 1)
    S1(c0, c2);
  for (int c1 = 1; c1 < c0; c1 += 1)
    S4(c1, c0);
  for (int c2 = c0 + 1; c2 <= M; c2 += 1)
    for (int c3 = 1; c3 < c0; c3 += 1)
      S3(c0, c2, c3);
}