M. Gyssens, und J. Paredaens. Proc. of the 3rd Symposium on the Principles of Database Systems (PODS 1984), Seite 142--152. Waterloo, Ontario, Canada, (April 1984)
M. Gyssens, und J. Paredaens. Advances in Data Base Theory, Vol. 2, Based on the Proceedings of the Workshop on Logical Data Bases (ADBT 1982), Seite 85--122. Toulouse, France, Centre d'études et de recherches de Toulouse, (1982)