I allow contributors to sign up on my site & post. I'd like to allow them to create categories and category descriptions. But when composing a post in their "post page" (and their other contributor pages) I'd like them to only see the categories that they themselves create --- and only be allowed to categorize their posts under categories that they themselves create. Well, plus three or four others that I create and allow all users to post to. Thoughts on how? I just don't want them to be able to stick their own posts under other users' categories.