R. Ducournau, and M. Habib. Proceedings of the European Conference on Object
Oriented Programming, special issue of Bigre No. 54, page 291--300. Paris, (1987)
L. Cardelli. in Semantics of Data Types, pages 51--67, G. Kahn, D.B. McQueen and G. Plotkin editors, Springer-Verlag, Lecture Notes in Computer Science, Vol. 173, (1984)