bless [ $skv, undef, undef, $m ], $cls;
}
-# returns true on unseen messages according to the deduplication strategy,
-# returns false if seen
+# returns true on seen messages according to the deduplication strategy,
+# returns false if unseen
sub is_dup {
my ($self, $eml, $oid) = @_;
!$self->[1]->($eml, $oid);