[Exim] filter rule precedence

I'm a little confused about the precedence of a filter rule I'm
trying to implement. I need messages addressed to 'tig-announce'
to be discarded unless they are explicitly from either of two
users, 'dlt' or 'rob@emsh'. This is what I wrote, and in the system
filter file it seemingly discards all mail:

if $local_part contains "tig-announce" and
$header_from does not contain "rob@emsh" or
$header_from does not contain "dlt"
then
seen finish
endif


...thanks in advance for any help.
--Rob

--
Rob Lingelbach     http://www.alegria.com       rob@???
 kb6cun     The whole world's not crazy.  --yiddish saying