How can I list all installed packages from lenny-backports?

In Debian or one of its derivatives, how can I list all packages which I have installed from lenny-backports?

The closest I've come up with is:

aptitude search ~i~Alenny-backports

However, that lists all installed packages for which there exists a version in backports even if the currently installed version is not the one from backports. I'd like to list only the packages for which the currently installed version is from backports (or, not lenny, if that's easier).

Cheers

Aptitude has a ?narrow search term for just this:

aptitude search '~S ~i ~Abackports'
aptitude search '?narrow(~i, ~Abackports)'