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