Refinement and Reachability in Event_B
Since the early 90’s (after the seminal article of R. Back ), the refinement of stuttering steps  are performed by means of new actions (called here events) refining skip. It is shown in this article that such a refinement method is not always possible in the development of large systems. We shall instead use events refining some kind of non-deterministic actions maintaining the invariant (sometimes called keep). We show that such new refinements are completely safe. In a second part, we explain how such a mechanism can be used to express some reachability conditions that were otherwise expressed using some special temporal logic statements à la TLA  in a previous article . Examples will be used to illustrate our proposals.
KeywordsRefinement Stuttering Reachability B Method
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