[EXIM] ${if def:xxx ...} in filter files?

I can't seem to get the ${if def:xxx ...} condition to work in filter files.
For example:

% cat ~/xx
# Exim filter

testprint ${if def:X-RBL-Warning {spam}}

% /usr/local/libexec/exim -bf xx < test-message
Testing Exim filter file /CS/system/ephraim/xx
Filter error: unknown filtering command "def:X-RBL-Warning" near line 3 of filter file
%

What am I doing wrong?

Thanks,
    Ephraim
___________________________________________________________________________
Ephraim Silverberg, CS System Group,          Phone number:   972-2-6585521
Hebrew University, Jerusalem, Israel.         Fax number:     972-2-6585439
WWW: http://www.cs.huji.ac.il/~ephraim        E-mail: ephraim@???

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