If the label of a named consequence is used right after a 'not' pattern, a compilation error is raised.
For example:

rule R1 when
$a: Cheese ( type == "stilton" )
not Cheese ( type == "brie" )
do[t1]
b: Cheese ( type == "cheddar" )
then
results.add( $b.getType() );
then[t1]
results.add( $a.getType() );
end