[Exim] headers or not in filters

hi,

i'm trying to arrange a filter (system filter, in fact) to catch
messages like this:

    From MAILER-DAEMON Mon May 27 19:40:37 2002
    Envelope-to: some@???
    Received: from innocent.protected
        helo=green-flash)
        by another.innocent with smtp (Exim 3.33 #2 (Debian))
            id 17COTs-0006Ad-00
        for <some@???>; Mon, 27 May 2002 19:40:28 +0200
    MIME-Version: 1.0
    Content-Type: multipart/mixed; boundary="--VEH2N4TIV4PMZ4XE3GX"
    Message-Id: <E17COTs-0006Ad-00@somewhere>
    Bcc:
    Date: Mon, 27 May 2002 19:40:28 +0200
    Delivered-to: innocent.here@again

in words, there are no From and Subject headers.

my question is: in this case, can i do

    if $header_subject: matches ^\\s*\$ and $header_from: matches ^\\s*\$

or, since there are in fact no such headers, do i need to do something
else (like if not defined $header_from:) ? if the latter, how ? i see
no reference to check whether a header exists or not. might be
overlooking, though..

thanks a lot,

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