> Since the collection of ISBNs can be treated as an equivalence class, > can't any arbitrary member of the class be designated as the group > identifier? This eliminates the need to create a synthetic id, and it > means that, for singular items, there's no need to create a separate > group id. > > In fact, assuming that a library has a single item, that ISBN is > designated as the "group id" for the group of one thing. > Then, when the > library acquires a second item that should be part of the group, it > should be easy to update the equivalence class. This is a very good idea. I'll see if I can come up with a case that would confound it, but I think you may have hit the nail on the head. Ben