  Hannestad and Scherrer analysed self-interacting Warm and Hot Dark Matter and
found power spectra in disagreement with previous papers. We argue that they
include self-interactions in a weak-coupling approximation, as for photons
after recombination. The ``tightly coupled'' approximation used for
pre-recombination photons, and previous discussions of self-interacting HMD,
would have been more appropriate. Their approximation generates a Boltzmann
hierarchy leading to a stiff system of equations. Furthermore contamination by
gauge modes could invalidate their results.
