restrict branching to allow free tag associated to parent items (state/city)

Per our email in the CCK Address forum, it would be great to have a 'restrict branching' feature. I want users to be able to select multiple states, and either select and/or free tag multiple cities -- some related to states, some not -- and assign new cities they add to a parent state (but not to a parent city term).

In other words, users can free tag to existing branches only, and not branch off existing siblings. "Your vocab would be the states, each would be a branch, and free tags could be attached to any of them. Cities would lie under each state branch and would not be allowed to have free tags connected to them."

This is brilliant module, looking forward to this patch -- thanks.