several possible types of children nodes + mutual exclusion of children types

Hi,
it would be nice to be able to set:
- more than one possible content type children of a certain type of node
- to have those children content type mutually exclusive

ie.
for our project we'd have an outline like below, summarized by the sentence: "Contributors can have programs split into several levels of categories, where each category can either have as children: only sub-categories or only programs and programs may not have any kind of children.".

[outline root]
         |
         |--------- cck nodes of type "category" 
                               |  ------- only cck nodes of type "program"
                               |
                    (or mutually exclusively)
                               |
                               | ------- only cck nodes of type "category"
                                                             |
                                                             |--------- only cck nodes of type "program"
                                                             |
                                                   (or mutually exclusively)
                                                             |
                                                             | ------- only cck nodes of type "category"
                               

(shown here: a tree with 2 levels max of categories which is what we want for our project... though let us say that we have no constraint on the number of levels)

jonathan