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