G. Greco, S. Greco, and E. Zumpano. Proceedings of the 17th International Conference on Logic Programming, page 348--364. London, UK, UK, Springer-Verlag, (2001)
M. Arenas, L. Bertossi, and J. Chomicki. Proceedings of the 18th Symposium on Principles of Database Systems, page 68--79. New York, NY, USA, ACM, (1999)