  Properties of Bosonic linear (quasi-free) channels, in particular, Bosonic
Gaussian channels with two types of degeneracy are considered.
  The first type of degeneracy can be interpreted as existence of noise-free
canonical variables (for Gaussian channels it means that $\det\alpha=0$). It is
shown that this degeneracy implies existence of (infinitely many) "direct sum
decompositions" of Bosonic linear channel, which clarifies reversibility
properties of this channel (described in arXiv:1212.2354) and provides explicit
construction of reversing channels.
  The second type of degeneracy consists in rank deficiency of the operator
describing transformations of canonical variables. It is shown that this
degeneracy implies existence of (infinitely many) decompositions of input space
into direct sum of orthogonal subspaces such that the restriction of Bosonic
linear channel to each of these subspaces is a discrete classical-quantum
channel.
