Madsen, OL (1989). "Virtual classes: A powerful mechanism in object-oriented programming". Conference proceedings on Object-oriented programming systems, languages and applications - OOPSLA '89. pp. 397–406. doi:10.1145/74877.74919. ISBN0897913337. S2CID1104130.
Cook, William R.; Hill, Walter; Canning, Peter S. (1990). Inheritance is not subtyping. Proceedings of the 17th ACM SIGPLAN-SIGACT Symposium on Principles of Programming Languages (POPL). pp. 125–135. CiteSeerX10.1.1.102.8635. doi:10.1145/96709.96721. ISBN0-89791-343-4.
Cook, William R.; Hill, Walter; Canning, Peter S. (1990). Inheritance is not subtyping. Proceedings of the 17th ACM SIGPLAN-SIGACT Symposium on Principles of Programming Languages (POPL). pp. 125–135. CiteSeerX10.1.1.102.8635. doi:10.1145/96709.96721. ISBN0-89791-343-4.
Madsen, OL (1989). "Virtual classes: A powerful mechanism in object-oriented programming". Conference proceedings on Object-oriented programming systems, languages and applications - OOPSLA '89. pp. 397–406. doi:10.1145/74877.74919. ISBN0897913337. S2CID1104130.