Topic on Extension talk:CategoryTree

I have pages with categories:

1. A and B
2. A and B
3. A and C

How to group pages with A category by other categories? Any ideas?

<categorytree mode="pages">A</categorytree>

Display:

A
   B
       1
       2
   C
       3

I don't know if an extension like this exists, but CategoryTree isn't one of them