Saake, G., R. Jungclaus, and C. Sernadas,
"Abstract Data Type Semantics for Many-Sorted Object Query Algebras",
Proceedings 3rd. Symp. on Mathematical Fundamentals of Database and Knowledge Base Systems MFDBS-91, Rostock (D): LNCS 495, Springer, Berlin, pp. 291-307, 1991.
Abstract