  The Conti-Boston factorization theorem (CBFT) for linear tail-biting trellis
realizations is extended to group realizations with a new and simpler proof,
based on a controller granule decomposition of the behavior and known
controllability results for group realizations. Further controllability results
are given; e.g., a trellis realization is controllable if and only if its top
(controllability) granule is trivial.
