{ diff(x1, t) = 0,
  diff(x2, t) = (1*(k9*k2 - x2*(k1 + k3*x4)) + (-1)*((-k6*x3) + k7*x2) + (-1)*(k5*x2 - k4*x4))/k8,
  diff(x3, t) = 1*((-k6*x3) + k7*x2)/k8,
  diff(x4, t) = 1*(k5*x2 - k4*x4)/k8 }