for (int c0 = 1; c0 <= M; c0 += 1)
  for (int c1 = 1; c1 <= M; c1 += 1)
    for (int c2 = 1; c2 <= M; c2 += 1)
      for (int c3 = 1; c3 <= M; c3 += 1)
        S1(c1, c2, c0, c3);