aah, now I understand it fully (probably a language thing.. :-)

you want to know distribution of categories in the result set, *not* in 
the whole collection, right? wow, that would be nice


Theoretically speaking am I completely
> wrong here?  If not, how hard would be it be to do this and could it be
> added to a TODO list somewhere?  If I am wrong, sorry for beating this
> dead horse.

I have seen this sort of thing in oclc search and in vivisimo
http://vivisimo.com/search?tb=homepage&query=swish-e&v%3Asources=Web