How to list all packages in a given category?

[Chris Pressey]

Hi all,

Apologies in advance if this is a stupid question, but I couldn't find
anything relevant in the man pages or with Google:

How can I list all the packages I have installed in a certain category?

I'm thinking it should be something like 'pkg_info -C games', but AFAICT
pkg_info doesn't have any such option like that.

TIA,
-Chris