[Exim] filter syntax problem

I have pored over the filter document for ages and can't work out what I am doing
wrong. I am sure it is quite a simple mistake, but I just can't work it out!

I want to test if a header is present and if so if it is blank, so I have

if ${if def:header_bcc:{bcc}{nobcc}} is "bcc" then .... endif

When I test using -bF I get "unrecognised condition word "def:header_bcc" near
line .."

This is particulary odd because the filter document on page 23 (version 3.30)
gives an example using def:header_reply-to:.

Phil.
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Phil Chambers (postmaster@???)
University of Exeter