{ diff(x1, t) = (1*k20*k1 + 1*k20*k19 + (-1)*k20*k2*x1^k3/(k4^k3 + x1^k3) + 1*k21*k5*x2^k6*x1^k11/((k7^k6 + x2^k6)*(k8^k11 + x1^k11)) + 1*k21*k9*x2 + (-1)*k20*k10*x1)/k20,
  diff(x2, t) = (1*k20*k2*x1^k3/(k4^k3 + x1^k3) + (-1)*k21*k5*x2^k6*x1^k11/((k7^k6 + x2^k6)*(k8^k11 + x1^k11)) + (-1)*k21*k9*x2)/k21,
  diff(x3, t) = 0/k20,
  diff(x4, t) = 1*k20*k14/x3*(k13*x1^k16/(k15^k16 + x1^k16)/k14*(1 - x4)/(k17 + 1 - x4) - x4/(k18 + x4))/k20 }