I've been working on the repr code a bit lately, and it's becoming clear to me that the veto callbacks probably aren't such a good idea.

1. they aren't sufficient to implement document validation constraints (which was their original purpose)

2. they would get in the way of some useful multi-step transformations where the end result is valid but the intermediate steps are not

3. they aren't used anyway except in one place (manually setting element IDs)

4. the one place they are used, it would make more sense to accomodate the user's wishes (by re-iding the object whose ID has been taken) than to silently disobey them as we do now

5. they get icky with DOM

So I'm working on removing them.

-mental