S = 4*S - 3*S
  = (x21 + x22 + x23)
   +(x11 + x22 + x33)
   +(x12 + x22 + x32)
   +(x13 + x22 + x31)
   -(x11 + x12 + x13)
   -(x21 + x22 + x23)
   -(x31 + x32 + x33)
  = 3*x22