TOWARDS HIERARCHICAL DESCRIPTION OF SYSTEMS: A PROOF SYSTEM FOR STRONG PREFIXING

Abstract

The problem of relating system descriptions at different levels of abstraction is addressed in the context of process description languages. As a case study, we introduce two nondeterministic languages. The latter is a simple extension of the former and is obtained by adding to its signature an operator of strong prefixing for making atomic the execution of a sequence of actions. The two languages are intended to be a specification and an implementation language, respectively. To directly relate them, we introduce a mapping, called atomic action refinement, from actions of the former to atomic sequences (i.e. sequences of actions built with strong prefixing) of the latter, which can be homomorphically extended to become a mapping among process terms of the two languages. A notion of implementation, based on a sort of bisimulation (parametric with respect to an atomic action refinement), relates processes of the two languages. Given a specification process P and an atomic action refinement ρ, the refined process ρ(P) is proved to be an implementation of P. Moreover, two complete proof systems for the two languages (and thus also for the operator of strong prefixing) are presented and proved consistent with respect to refinement: if P and Q are congruent processes of the specification language, then ρ(P) and ρ(Q) are congruent, too.