Unidentified subject!

I have a lot of (spam-guard) recipes like this:

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* ^(From.|Reply-To:|Message-ID:|Received:).*badhost.com

I've seen (I think) a way to put the various Headers into a variable, so I
could do something like this:

SpamHeaders="From|Reply-To:|Message-ID:|Received:"

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* ^$SPAMHEADERS.*badhost.com

but there was some type of 'gotcha' I can't remember and can't find any
reference to....

Clues?

TjL