{ diff(x1, t) = 0,
  diff(x2, t) = (1*k2*k3*k8 + (-1)*k2*x4*k5*x2/(k4 + x2) + (-1)*k2*k7*x2)/k2,
  diff(x3, t) = (1*k2*x4*k5*x2/(k4 + x2) + (-1)*k2*k6*x3)/k2,
  diff(x4, t) = 0/k2 }