Formal Aspects of Computing (1999) 11: 472–473c© 1999 BCS Formal Aspects

of Computing

‘The Standard Logic for Z’: A Clarification

Steve KingDepartment of Computer Science, University of York, UK

Keywords: Specification languages; Specification logic

Abstract. Some clarification is made of a recent article in Formal Aspects ofComputing [Hen98]. Specifically, it is noted that the inconsistency found is in adraft logic, which is clearly unfinished. The latest news of the position of the ZStandards Panel on a logic for Z is given.

A recent issue of Formal Aspects of Computing contained a paper by MartinHenson, entitled ‘The Standard Logic of Z is Inconsistent’ [Hen98]. There areseveral points of clarification which are worth making, to avoid the possibilityof misunderstandings arising from this paper. The standardisation process for Zis (still) not complete, and so there is no such object as ‘the Standard Logic ofZ’, whether consistent or not. The logic to which Henson refers was publishedin September 1995 as Annex F of the first Committee Draft (CD) of the ZStandards Panel [Nic95]. It has long been known by its authors and others tobe unsatisfactory in many ways. For instance, a careful reading shows that theUniqExists, Scomp and Sdecor inference rules all contain the rather unhelpfulplaceholder SOMETHING, and there are several other pointers to the fact thatthis is an unfinished draft.

Much work has taken place since the first CD was published and a second CDhas been submitted for comment during the summer of 1999. This has correctedmany of the mistakes, ambiguities and internal inconsistencies of the first CD,and it is hoped that it will form the basis of a Draft International Standard,and, eventually, an International Standard. The position of the Panel is thatthere should be no ‘logic for Z’ in the latest CD: since the standard defines thesemantics of Z specifications, this is sufficient for determining the soundness andconsistency of any set of inference rules. There is therefore no requirement tostandardise on a particular set of rules.

Correspondence and offprint requests to: Steve King, Department of Computer Science, University ofYork, Heslington, York, YO10 5DD, UK. email: king@cs.york.ac.uk

‘The Standard Logic for Z’: A Clarification 473

Thus, though Henson’s recording of one of the many problems of the draftlogic of the first CD is to be welcomed, it is unlikely that he will find any fault inthe inference rules of the second CD, or, it is hoped, in the rules of the eventualInternational Standard for Z.

References

[Hen98] Henson, M.C.: The standard logic of Z is inconsistent. Formal Aspects of Computing,10(3): 243–247, 1998.

[Nic95] Nicholls, J.E. (ed.) Z Notation – First Committee Draft. ISO, September 1995.