Søren Prehn W. J. Toetenel VDM '91 - Formal Software Development, 4th International Symposium of VDM Europe, Noordwijkerhout, The Netherlands, October 21-25, 1991, Proceedings, Volume 1: Conference Contributions VDM Europe (1) Lecture Notes in Computer Science 551 Springer 1991 3-540-54834-3 db/conf/fm/vdme1991-1.html