  We derive a nonlinear equation governing dynamics of short-wavelength
longitudinal waves in ultrarelativistic electron-positron-ion plasmas. In
contrast to the recent work by Lashkin [Phys. Plasmas {\textbf{27}}, 102302
(2020)], where a similar equation was suggested in the framework of the Wigner
function approach for a nonrelativistic electron-ion degenerate plasma, in our
case which is based on the Vlasov kinetic equation all three species of
particles (electrons, positrons and ions) should be present. The nonlinearity
arises only in the presence of a population of ions. By numerical simulations
we demonstrate that collisions between even four solitons are fully elastic.
