formail: how to prefix subject?

On building an autoreply facility with procmail/formail, I
am wondering how to generate a subject line which
includes the original subject using (was: old subject) or similar?

ie, to a mail with 
        Subject: Hello
I want to reply with:
        Subject: Thanks (was: Hello)

Does anyone have an idea how to do this using formail?

If not I'll have to fall back to perl...

Any help is appreciated

Mathias Koerber   | Tel: +65 / 471 9820    |   
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