  Recent inelastic neutron scattering experiments on (VO)_2P_2O_7 (VOPO) have
seen previously unanticipated sharp peaks in the dynamic structure factor in
addition to the pair of triplet modes observed earlier. We argue that the
additional features are in essence `shadows' of the previously studied features
arising due to umklapp scattering, and suggest experimental tests of this
proposal. The basic point is illustrated by some elementary calculations within
a strong-coupling expansion for the alternating chain with the right geometry
(on which the matrix element for umklapp scattering depends sensitively) taken
fully into account.
