I consider DU chains different from UD chains, and the article as written seems to share that view. However, I've noticed a tendency in the literature to conflate the two, or consider them to be completely interchangeable. I wonder if this deserves some mention in the article. --Mike Van Emmerik 23:05, 3 October 2006 (UTC)
What do you mean by different? They are obviously very closely related in many ways. I don't see how they are interchangeable, in that they provide different kinds of information to provide answers to different questions. Derek farn 10:02, 4 October 2006 (UTC)