> 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