{ diff(x1, t) = (1*k13*k1 + (-1)*k13*k4*x1*x2^k10/(k7^k10 + x2^k10))/k13,
  diff(x2, t) = (1*k13*k3*(1 - x2)*x3^k12/(k9^k12 + x3^k12) + (-1)*k13*k6*x2)/k13,
  diff(x3, t) = (1*k13*k2*(1 - x3)*x1^k11/(k8^k11 + x1^k11) + (-1)*k13*k5*x3)/k13 }