A General Datalog−Based Framework for Tractable Query Answering over Ontologies (Extended Abstract) - Calì, A; Gottlob, G; Lukasiewicz, T (Proceedings of the 17th Italian Symposium on Advanced Database Systems‚ SEBD 2009‚ Camogli‚ Italy‚ June 21−24‚ 2009, 2009)