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)
M. Arenas, L. Bertossi, and J. Chomicki. PODS '99: Proceedings of the eighteenth ACM SIGMOD-SIGACT-SIGART symposium on Principles of database systems, page 68--79. New York, NY, USA, ACM, (1999)