[Exim] I'm missing something! I must be....

I'm having a problem with this piece of code in an exim filter file:

# Marked as Spam by RBL
if
#         ${if def:h_X-RBL-Warning: {$h_X-RBL-Warning:}}
           ${if def:header_reply-to:{$h_reply-to:}{$h_from:}}
then
       deliver spamcatcher
       logwrite "- XBL marked this as SPAM"
       finish
endif

Exim is 3.36 on Debian Sarge. Someone kindly helped me out with a
couple of syntax errors, but the code still fails with:

--------------------------------------------------------------------------------------
Error in message_filter file: unrecognized condition word
"def:header_reply-to:{$h_reply-to:}{$h_from:}}" near line 17 of filter
file
--------------------------------------------------------------------------------------

I've tried everything. I've even cut-and-pasted the sample line from
the filter docs for 3.36 and I still get that error (as shown above).

Does def:header not work? Or, more likely, am I missing something
obvious?

Cheers,

Cliff
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