[ x[17] = 3/2 - x[16],
  x[21] = 15 - 2*(x[1] + x[3] + x[6] + x[9]) - (x[2] + x[4] + x[5] + x[7] + x[8] + x[10] + x[11] + x[12] + x[13] + x[14] + x[15] + x[16]) ]