with the Dirac gamma matrices $\gamma^{\mu}$ and where the symbol $\circlearrowleft$ indicates the amplitude corresponding to the diagram on the right in Fig. 1. In Eq. (\[M\]) the quantity $G(x,y|A)$ is the dressed electron propagator in the laser field. The latter is described by the four-vector potential $A^{\mu}=A^{\mu}(\phi)$, where $\phi=(nx)$, with $n^{\mu}=(1,\bm{n})$ and $\bm{n}$ being the propagation direction of the laser field. By working in the Lorentz gauge, the four-vector potential $A^{\mu}(\phi)$ of the laser field can be chosen in the form $A^{\mu}(\phi)=(0,\bm{A}(\phi))$, with $\bm{n}\cdot\bm{A}(\phi)=0$. Let $\bm{a}_1$ and $\bm{a}_2$ indicate the two possible independent laser polarization directions, such that $\bm{a}_r\cdot\bm{a}_s=\delta_{rs}$, with $r,s=1,2$, and that $\bm{a}_1\times\bm{a}_2=\bm{n}$. Then, the four-vector potential $A^{\mu}(\phi)$ can be written as $A^{\mu}(\phi)=A_0[a^{\mu}_1\psi_1(\phi)+a^{\mu}_2\psi_2(\phi)]$, where $A_0=-E_0/\omega_0$, $a^{\mu}_r=(0,\bm{a}_r)$, and the two shape-functions $\psi_r(\phi)$ are arbitrary, smooth functions except that they satisfy the relation $\sqrt{\psi^{\prime 2}_1(\phi)+\psi^{\prime 2}_2(\phi)}\le 1$ for all values of $\phi$, with $\psi'_{1/2}(\phi)=d\psi_{1/2}(\phi)/d\phi$. Here, $E_0$ and $\omega_0$ indicate the laser-electric-field amplitude and its central angular frequency, respectively[^3]. Since the interaction of the $j$th photon with the laser field is controlled by the parameter $\varkappa_j=\eta_j\xi$, with $\eta_j=\omega_0k_{j,X}/m^2$ [@Ritus_1985; @Di_Piazza_2012], it is natural to assume here that $\varkappa_j\ne 0$ for all $j$s, which means $k_{j,X}\ne 0$ for all $j$s. This means 