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