[ Eq(x4, x1 - x2),
  Eq(x6, k18 - x3),
  Eq(x9, 1/2*(x2 - k19 - k2/k3*(k3 + x3 + k6*(k18 - x3)) + sqrt((k2/k3*(k3 + x3 + k6*(k18 - x3)) - x2 + k19)**2 + 4*x2*k2/k3*(k3 + x3 + k6*(k18 - x3))))),
  Eq(x10, 1/2*(x1 - k20 - k4 + sqrt((k4 - x1 + k20)**2 + 4*x1*k4))),
  Eq(x12, (k2 + x9)/(k2/k3*(k3 + x3 + k6*(k18 - x3)) + x9)),
  Eq(x13, x2/(k5 + x1)),
  Eq(x14, x1/(k5 + x1)),
  Eq(x15, (x1 - x2)/(k5 + x1)),
  Eq(x16, x3/k18) ]