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