P. Lyngbaek, and V. Vianu. Proceedings of the 12th Annual ACM Conference on the Managemant of Data, page 132--142. San Francisco, California, (May 1987)
M. Carey, D. DeWitt, and S. Vandenberg. Proceedings of the 13th Annual ACM Conference on
the Management of Data, page 413--423. Chicago, Illinois, (June 1988)