We are grateful to G.David Forney,Jr. for suggesting some of the problems
addressed in this work. We would also like to thank Frank Kschischang for sending us preprints of his papers and
Alexander Vardy for motivating this work, providing important feedback, and suggesting
Methods 1 and 2. Finally, we would like to thank
the anonymous referees for their valuable suggestions for tightening the
presentation of this paper.