  Analysis of electromagnetic planewave propagation in a medium which is a
spatiotemporally homogeneous, temporally nonlocal, isotropic, chiral medium in
a co-moving frame of reference shows that the medium is both spatially and
temporally nonlocal with respect to all non-co-moving inertial frames of
reference. Using the Lorentz transformations of electric and magnetic fields,
we show that plane waves which have positive phase velocity in the co-moving
frame of reference can have negative phase velocity in certain non-co-moving
frames of reference. Similarly, plane waves which have negative phase velocity
in the co-moving frame can have positive phase velocity in certain
non-co-moving frames.
