[ Eq(Derivative(x4, t), 0),
  Eq(Derivative(x1, t), x1*(k1 - k3*x1 - k5*x2) - k13*x1*k20),
  Eq(Derivative(x2, t), x2*(k11 - k4*x2) - k6*x3*x2 + k14*x1*k20),
  Eq(Derivative(x3, t), k7 + k8*x3*x2/(k9 + x2) - k12*x3*x2 - k10*x3 - k15*x3*k20/(k16 + k20)) ]