{ diff(x1, t) = (1*k8*(k1*k6*x1 - k2*x3*x1) + (-1)*k8*k4*x1)/k8,
  diff(x2, t) = (1*k8*k4*x1 + (-1)*k8*k5*x2)/k8,
  diff(x3, t) = (1*k8*k5*x2 + (-1)*k8*k3*x3)/k8,
  diff(x4, t) = 1*k8*k7*x3*x1*x4/k8 }