 \frac{0}{0} \int \varphi_{a}^{\dagger}(x_0) \varphi_{b}^{\dagger}(x_0) \varphi_{a}(x_0)\varphi_b(x_0) + \frac{0}{0} \int \varphi_{a}^{\dagger}(x_0) \varphi_{b}^{\dagger}(x_0)\varphi_{b}(x_0)\varphi_a(x_0) \\ &