{ diff(x1, t) = (1*k10*k1/(1 + x2^k3) + (-1)*k10*k11*x1)/k10,
  diff(x2, t) = (1*k10*k2/(1 + (x1/(1 + x3/k8)^k9)^k4) + (-1)*k10*k12*x2)/k10,
  diff(x3, t) = 0/k10 }