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