procmail with virtual mailboxes and postfix

[d0wn_under]

Replacing ubuntu with gentoo, I've just followed this guide to setup a mail system:
http://flurdy.com/docs/postfix/

and now I'd like to get procmail working with the virtual mailboxes.

There is a space for the procmailrc filename in the database but there is no mention of how to use it. I've tried asking in the ubuntu forum but got no answer. Can anyone here help me?
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