Jean-François Baget Michel Leclère Marie-Laure Mugnier Eric Salvat Extending Decidable Cases for Rules with Existential Variables. 677-682 2009 IJCAI http://ijcai.org/papers09/Papers/IJCAI09-118.pdf conf/ijcai/2009 db/conf/ijcai/ijcai2009.html#BagetLMS09