{ diff(x1, t) = 0,
  diff(x2, t) = (1*k2*k3*k9 + (-1)*k2*k6*x2 + (-1)*k2*(k7*x2*x4 - k8*x5))/k2,
  diff(x3, t) = (1*k2*k4*x5 + (-1)*k2*k5*x3)/k2,
  diff(x4, t) = (1*k2*k4*x5 + (-1)*k2*(k7*x2*x4 - k8*x5))/k2,
  diff(x5, t) = ((-1)*k2*k4*x5 + 1*k2*(k7*x2*x4 - k8*x5))/k2 }