[ Eq(1*x1 + 1*x2 + 1*x3 + 1*x6 + 1*x7 + 1*x8 + 1*x9 + 1*x10 + 1*x11, k18),
  Eq(1*x4 + 1*x6 + 1*x7, k19),
  Eq(1*x5 + 1*x8 + 1*x9 + 1*x10 + 1*x11, k20) ]