{ x15 = x1 + x4,
  1*k11*x1 + 1*k11*x2 + 1*k11*x3 + 1*k11*x4 + 1*k11*x5 + 1*k11*x6 + 1*k11*x7 + 1*k11*x8 = k11*k18 }