UseCase-Wise Development: Retrenchment for Event-B

UseCase-wise Development, the introduction of functionality into an application in stages, with each stage being carried through to (ideally) implementation before the next is considered, is examined with a view to its being treated via an Event-B methodology. The need to modify top level behaviour in a non-skip way precludes its naive treatment via Event-B refinement, and paves the way for the use of retrenchment in Event-B. The details of an Event-B formulation of retrenchment, aligned to the practical details of the Rodin toolset, are described. The details of refinement/retrenchment interworking needed to handle UseCase-wise development are outlined, and a simple case study is given.