[Exim] system filter syntax

I don't quite understand how to present an "if def" in my system filter.

I want to re-direct email incoming with a header using X-UIDL,
and the following:

if ${if def:h_X-UIDL}
   then deliver spam_grabber
        finish
endif

generates a syntax error. What have I done wrong?

Also, is it redundant to include the "finish" command as shown above?

TIA
--
Jim Pazarena    mailto:paz@qcislands.net
                http://www.qcislands.net/paz