  Based on the invariance of the phase of waves, plane waves was shown to
propagate with negative frequencies in a medium which moves at superluminal
speeds opposite to the propagation direction of plane waves. The validity of
the invariance of the phase of plane waves was then called into question. A
radical change of the conventional concept of plane waves is recently proposed
to solve the problem of negative frequency of waves. Here, we point out flaws
in that proposal. Thus, the validity of the invariance of the phase of plane
waves remains questionable.
