Stepwise Development from Ideal Specifications

The stepwise development of a program using reene-ment requires that the original abstract speciication is realisable, i.e., an implementation exists with identical functionality. In some situations, this may not be desirable or even possible, and an ideal speciication which is only approximated by the nal implementation is used. For these speciications, an informal step, based on the developer's knowledge and experience, is typically used during the reenement process in order to transform the speciication to one which is realisable. This paper introduces a formal approach to such speciication transformations called realisation. It enables a speciication to be transformed to another with diierent functionality and, at the same time, allows properties of the new speciication to be derived from those of the original.