  Two families of sets, nonstationary and stationary, are obtained. Each
nonstationary set $\psi_{p_v}$ consists of the solutions with the quantum
number $p_v=p^0v-p_3.$ It can be obtained from the nonstationary set
$\psi_{p_3}$ with quantum number $p_3$ by a boost along $x_3$-axis (along the
direction of the electric field) with velocity $-v$. Similarly, any stationary
set of solutions characterized by a quantum number $p_s=p^0-sp_3$ can be
obtained from stationary solutions with quantum number $p^0$ by the same boost
with velocity $-s$. All these sets are equivalent and the classification (i.e.
ascribing the frequency sign and in-, out- indexes) in any set is determined by
the classification in $\psi_{p_3}$-set, where it is beyond doubt.
