{ x10 = x1 + x3,
  x11 = x2 + x4,
  x12 = x8 + x3 + x4,
  x15 = x13 + x14,
  x16 = x7 + x13 + x14,
  x17 = x9 + x13,
  x18 = x9 + x6 + x13 + x14,
  1*k27*x6 + 1*k27*x9 + 1*k27*x13 + 1*k27*x14 = k27*k28 }