How does aptitude record automatically installed packages?

[Simon Guest <simon@guests.plus.com>]

A great feature of aptitude is that it tracks automatically installed
packages, removing these when no package I have explicitly installed
depends on them.  But where does it record this info?

I want to be able to see what I have explicitly installed ('manually
installed') on my system, since this is the list I have to manage to keep
my system maintainable.  Writing it down in a logbook just doesn't cut it.
 ;-)

I've RTFM'd, and stared hard at /var/lib/aptitude/pkgstates, but can't
work it out.

Anyone?

cheers,
Simon

PS: What I really want is a script that lists what was manually installed.
 I expect to have to write that, unless someone can tell me differently.