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