{ diff(x1, t) = ((-1)*k1*x1 + 1*k5*x1)/k13,
  diff(x2, t) = (1*(k2 + k3*x1/(x1 + k4))*x1 + (-1)*k6*x2 + 1*k10*x2)/k13,
  diff(x3, t) = (1*(k7 + k8*x2/(x2 + k9))*x2 + (-1)*k11*x3)/k13 }