[Exim] logwrite local_part or original_local_part

In my system filter, I am unable to logwrite the $local_part or
$original_local_part.

I am able to logwrite other variables (so I know it works).

I saw one example of this in a March 2001 posting.

I am using Exim from Debian package 3.12-10.2.

Any ideas on how I can find out the $local_part or $original_local_part of
an incoming email?

Jeremy C. Reed