{ diff(x1, t) = (1*k5*k1*x1*(1 - x1) + (-1)*k5*k2*x1)/k5,
  diff(x2, t) = (1*k5*k3*x2*(1 - x2 - x1) + (-1)*k5*k4*x2 + (-1)*k5*k1*x2*x1)/k5 }