Messy ports, how to clean them up?

[O. Hartmann]

Well,
I use portsnap and portupgrade on a regular basis and therefore I could
watch very often the rebuild of ports - a nice and neat thing of FreeBSD.

Bit sometimes I or someone else installs ports an they install
dependencies and then he/she or I decide to kill/delete a specific port,
but very often dependencies remains on the system and doing this
deletion a couple of times will end in some 'zombie' remains of ports.

Is there a way cleaning up automatically a messy ports collection? Like
portupgrade does, only the opposite way, not rebuilding/reinstalling a
rebuilt/upgraded port, looking for stale ports never used anymore by
another port?

Thanks a lot in advance,
Oliver

P.S. I'm not very familiar with the complexicity of the pkgtoolset and
ports collection, sorry.

-- 
O. Hartmann