{ diff(x4, t) = 0,
  diff(x1, t) = x1*(k1 - k3*x1 - k5*x2) - k13*x1*k20,
  diff(x2, t) = x2*(k11 - k4*x2) - k6*x3*x2 + k14*x1*k20,
  diff(x3, t) = k7 + k8*x3*x2/(k9 + x2) - k12*x3*x2 - k10*x3 - k15*x3*k20/(k16 + k20) }