Overview: The proceedings of the fourth Vienna Development MethodSymposium, VDM ‘91, are published here in two volumes.Previous VDM symposia were held in 1987(LNCS 252), 1988(LNCS 328), and 1990 (LNCS 428).The VDM symposia have been organized by the VDM Europe,formed in 1985 as an advisory board sponsored by theCommission of the European Communities. The VDMEuropeworking group consisted of reasearchers, software engineers,and programmers, all interested in prommoting the industrialusage of formalmethods for software development. The fourthVDM symposium presented notonly VDM but also a large nu
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