A. Cal\`ı, G. Gottlob, and T. Lukasiewicz. Proceedings of the 12th International Conference on Database Theory, page 14--30. New York, NY, USA, ACM, (2009)
S. Schenk. KI 2007: Advances in Artificial Intelligence, volume 4667 of Lecture Notes in Computer Science, page 160--174. Springer Berlin / Heidelberg, (2007)