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