clean only old distfiles?

[Michael Dickens]

I've looked through port's help & manpage, and can't figure this
out easily: Is there a way to clean just old (not currently in
use) distfiles?  Doing "port clean --dist [list]" cleans both old
and current, and oftentimes a port just has a revision bump &
hence keeping the distfile around is convenient.  But, keeping
the old stuff just ends up wasting disk space -- hence, it would
be nice to easily be able to remove it.  Ideas? - MLD