-t ~.$$ We call it the $t$ -kinematics. According to the terminology introduced in [@ggfl], it is the forward kinematics (with respect to the charge flow) for $e^+e^- \to \mu^+ \mu^-$ and $e^+e^- \to d \bar{d}$. At the same time, it corresponds to the backward kinematics for $e^+e^- \to u \bar{u}$. In this kinematics, $\theta \ll 1$.

Second, there is the opposite kinematics where $\theta \sim\pi$ and therefore $$\label{ukin}
 s \sim -t \gg -u ~.$$

We define the configuration (\[ukin\]) as the $u$-kinematics. It corresponds to the forward scattering for $e^+e^- \to u \bar{u}$ and the backward scattering for $e^+e^- \to \mu^+ \mu^-$,  $e^+e^- \to d \bar{d}$.

To simplify the calculations, it is convenient to introduce the projection operators $(P_j)^{i i'}_{k' k} ~(j = 1,2,3,4)$, so that $A^{i i'}_{k' k}$ can be written in the following form: $$\label{invampl}
A^{i i'}_{k' k} = \frac{\bar{u}(-{p'}_2) \gamma_{\mu} u({p'}_1)
\bar{u}(-p_2) \gamma^{\mu} u(p_1)}{s}
|\Big[ (P_j)_{kk'}^{ii'} A_j + (P_{j + 1})_{kk'}^{ii'} A_{j + 1}\Big] ~,$$ where $j = 1$ for the $t$-kinematics and $j = 3$ for the $u$-kinematics. The representation (\[invampl\]) reduces the calculation of the matrix amplitude $A^{i i'}_{k' k}$ to the calculation of the invariant amplitudes $A_j$. The explicit expressions for the operators 