Well, killes commented in the issue that you will have problems with multiple inheritance. I just pointed out that with hook_taxonomy you could move this to a module. I was not thinking on other trees, but Dries is quite right that there are many single inheritance trees in Drupal (we talked a lot about them) AND many taxonomies are not multi inheritance -- they could benefit from this, too. I hope this clarifies what's happened so far.
I think it was the right path, too. What Dries wants out of this -- and I had not even thought about -- is a generic algorithm that can be used for any tree. It makes a lot of sense, but I was only focused on taxonomy because of what I was doing.
A little bit of contemplation is showing me that some thought is going to have to go into the design. I will post a draft on the forum later today for feedback. There's a couple of different ways to do this and I want to present them both (or all if I think of a third) and see what people think.
Thanks Earl. I hope it is possible to come up with a generic implementation that is elegant; it could significantly speed up the book module. Looking forward to your designs, -- Dries Buytaert :: http://www.buytaert.net/