R. Agrawal, and R. Srikant. Proceedings of the 20th international conference on Very Large Data Bases (VLDB'94), page 478--499. Morgan Kaufmann, (September 1994)
U. Dayal, A. Buchmann, and D. McCarthy. Lecture notes in computer science on Advances in object-oriented database systems, page 129--143. New York, NY, USA, Springer-Verlag New York, Inc., (1988)