  Bragg diffraction is comparable to a hard-wall reflection if the Bragg
condition is exactly fulfilled. However, in a neutron interferometer in the
gravitational field (COW experiment) this is not the case and the momentum
transfers should not be described by hard wall reflection, as it has been done
in the commented article. Instead, and quite similar to the atom case, each
mirror or beam splitter creates a constant momentum transfer given by the
reciprocal lattice vector of the Bragg crystal. To lowest order there are no
differences between the atom and the neutron case.
