{ x4 = x1 - x2,
  x6 = k18 - x3,
  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)))),
  x10 = 1/2*(x1 - k20 - k4 + sqrt((k4 - x1 + k20)^2 + 4*x1*k4)),
  x12 = (k2 + x9)/(k2/k3*(k3 + x3 + k6*(k18 - x3)) + x9),
  x13 = x2/(k5 + x1),
  x14 = x1/(k5 + x1),
  x15 = (x1 - x2)/(k5 + x1),
  x16 = x3/k18 }