Listing installed ports without any ports dependent on it

[Adam]

What I need to do is this:

For each installed port that has NO other installed ports dependent on
it, output the full name of the port

In other words, I want a script to generate the list of ports that can
be deinstalled without forcing (-f).

A few months ago a guy posted a python script to this mailing list that
did this, but I've since lost the script and it's too long ago for any
of the online archives.

If you're the nice guy that shared this script before, would you mind
sharing it again? Or, does someone else have a script to do this? I
remember it requiring a bit of cleverness to get it to work nicely.

Thanks,
-- 
Adam <blueeskimo at gmx.net>