In my mind there ought not be any difference, atleast as far as the datastore is concerned, between a "post" and a "category". That is, the interface for editing a category is exactly the same as the one used to create and edit post. Okay, maybe the standard <link> definition might be autogenerated to point back to itself. Similarly, the <title> might become fixed if the other widgets in your weblog chooses not to refer to other items via UIDs. But the point is that a "category" can have a "post" body (or an abstract and a body, and an excerpt and an abstract and body and so on and so on...) and "post" comments and "post" categories. The list of categories available to a category are exactly the same (minus itself) as those that a user may assign to a "post". What makes this interesting, is that the nested associations create themselves.