{ diff(x1, t) = 0,
  diff(x2, t) = (1*k2*k3*x3/(k4*(1 + x2/k5) + x3) + (-1)*k2*(k13/(1 + k21/k11)*x2/(k16*(1 + x3/k11) + x2) + k14/(1 + k21/k12)*x2/(k15*(1 + x3/k12) + x2)) + (-1)*k2*k19*x2/(k20 + x2))/k2,
  diff(x3, t) = ((-1)*k2*k3*x3/(k4*(1 + x2/k5) + x3) + 1*k1*(k21/(k6 + k21)*k7/(1 + x2/k10 + x3/k6) + k21/(k8 + k21)*k9/(1 + x2/k10 + x3/k8)) + (-1)*k2*k17*x3/(k18 + x3))/k2 }