Locating obsolete ports distfiles

[Peter Jeremy]

I currently have just over 8GB is /usr/ports/distfiles.  Some of these
files are more than 10 years old and long obsolete.  Does anyone have
any suggestions on how to identify which files are no longer referenced
by current ports?

Doing a 'make checksum' on every installed port and then looking at
the atimes is one approach but this doesn't handle:
- ports that I don't currently have installed but might need
- ports installed on systems that mount /usr/ports readonly

-- 
Peter Jeremy