How can I save a message to a file named by its subject ?

Hi, 
        I am unable to do this:

I want to save every message from `tweet` in a file containing
the subject of this message.  If the subject is `erv-vole-12-23-43`, I
want to save this message in a file `erv-vole-12-23-43.sgf`.

I am new with procmail, so please be explicit in your responses.  

        Thanks a lot,
                        Ervig, lapalmee(_at_)crm(_dot_)umontreal(_dot_)ca