[ x[21] = (x[18] + x[7] + x[19] + x[8])/2,
  x[22] = (x[18] + x[7])*k[16],
  x[23] = (x[19] + x[8])*k[17],
  x[24] = (x[7] + x[8])/2*k[18],
  x[25] = x[7]*k[19],
  x[26] = x[8]*k[20],
  x[27] = x[20],
  x[28] = (x[19] + x[8])/(x[18] + x[7]) ]