dovecot - Mar 2009 - deliver command line options "visible" in sieve script

I use dovecot mailbox as destination of fetches from a few POP
mailboxes. I would like to pass name of "source mailbox" to sieve
script
via deliver's command line option(s).

As I understand I should use -a "mailbox at domain".
How it would be accessible in sieve script?

-- 
[pl>en: Andrew] Andrzej Adam Filip : anfi at onet.eu
It is always the best policy to tell the truth, unless, of course,
you are an exceptionally good liar.
  -- Jerome K. Jerome