Formal semantics for Ward and Mellor's transformation schemas and its application to fault tolerant systems

A family of formal semantics is given for the Essential Model of the Transformation Schema of Ward and Mellor1 using recent techniques developed for defining the semantics of Statecharts2 by Pnueli and Huizing. A number of ambiguities and inconsistencies in Ward and Mellor's original definition is resolved. The models developed closely resemble those used for synchronous languages3. Each model has its own application area, e.g. one fits best for fault-tolerant systems.