removal of nomore-required packages

[davide zanon <d.zanon%infinito.it@localhost>]

hi!
I hope this question hasn't already been asked... how can I tell
the pkgsrc infrastructure to uninstall only and all the packages needed
by another package I don't want to use anymore?
Let me explain better: I installed kdebase and in the process lots of
other software got built and installed. Now I want to remove kdebase
and every package that was built for it and is not required by other
installed packages. Is there a possible way to do such a thing?
Thank you very much

Davide