Cleaning with aptitude

[Pupeno]

In a recent thread it was mentioned that aptitude can know what you
installed by hand and what was pushed as a dependency and that upon
removing a hand-installed packages, the non-needed dependencies would be
remove. A feature the Gentoo packaging system has and that I miss since
I switched to Ubuntu.
Now, is it possible to run aptitude tilling to remove those unneeded
dependencies ? 
I know it would trigger all since I never installed something with
aptitude, so I'll start marking what I care as hand installed and the
rest will be cleaned out.
I've read parts of the man page, but I couldn't find any option to do
that.
Thanks.
-- 
Pupeno <pupeno at pupeno.com> http://pupeno.com
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