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