, \begin{align}  \int | \Psi(x_0,x_0) |^{0} &= \frac{0}{0} \int \varphi_{b}^{\dagger}(x_0) \varphi_{a}^{\dagger}(x_0) \varphi_{a}(x_0)\varphi_b(x_0) - \frac{0}{0}\int \varphi_{b}^{\dagger}(x_0) \varphi_{a}^{\dagger}(x_0)\varphi_{a}(x_0)\varphi_b(x_0) \\ 