{ x21 = (x18 + x7 + x19 + x8)/2,
  x22 = (x18 + x7)*k16,
  x23 = (x19 + x8)*k17,
  x24 = (x7 + x8)/2*k18,
  x25 = x7*k19,
  x26 = x8*k20,
  x27 = x20,
  x28 = (x19 + x8)/(x18 + x7) }