Get list of packages installed from a given repo

[Mel Pilgrim]

I maintain local repos on top of the public pkg repo, and I want to 
check which pkgs were installed from which repos.  I know pkg knows this 
information (the %R query keyword), but this doesn't work:

# pkg query -e '%R !~ FreeBSD' %o
malformed evaluation string
# pkg query -e '%R ~ local' %o
malformed evaluation string

How do I query for a list of installed pkgs based on an evaluation of 
the repo name?