{ diff(x1, t) = 0,
  diff(x2, t) = (1*(k8*k2 - k1*x2) + (-1)*((-k5*x3) + k6*x2/(1 + x4)) + (-1)*((-k3*x4) + k4*x2/(1 + x4)))/k7,
  diff(x3, t) = 1*((-k5*x3) + k6*x2/(1 + x4))/k7,
  diff(x4, t) = 1*((-k3*x4) + k4*x2/(1 + x4))/k7 }