Liberalising Event B without changing it

Abstract We transfer a process algebraic notion of renement to the B method by usingthe well-known bridge between the relational semantics underlying the B machinesand the labelled transition system semantics of processes. Thus we dene deltarenement on Event B systems. We then apply this new renement to a problemfrom the literature that previously could only be solved by retrenchment. Keywords : process renement, automatic verication, frame renement, Event B 1 Introduction In this paper we will clearly state a well-known formal relation between the set theo-retic semantics underlyingB machines and the labelled transition system (LTS) seman-tics of process algebra. However since such operational semantics only dene part ofthe semantics, we also need to dene a renement relation on the operational seman-tics.The semantics (meaning) of a LTS is commonly given by dening a renementrelation on the LTS, indeed see Glabeck [1] for an interesting survey of renements andtesting semantics. The informal intuition is that the meaning of a specication is givenby the set of implementationsthat