Abstract. A local invariant is a set of states of a transition system with the property that every action possible from each of its states takes the system back into the local invariant, unless it is an ‘exit’ action, each of which is accessible from every state in the set via a sequence of non-‘exit’ actions. This idea supports a form of abstraction construction, abstracting away from behaviour internal to the local invariants themselves, that involving their non-‘exit’ actions. In this way, for example, it is possible to construct from a system one which exhibits precisely the externally visible behaviour. This abstraction is reminiscent of hiding in process algebra, and we compare the notion of abstraction with that of observational equivalence in process calculus.

Abstract. In this paper, we investigate the structure of the asynchronous behaviour of systems of cyclical, deterministic sequential processes. Specifically, we find conditions which, under certain finiteness assumptions, characterise periodic behaviours and pseudo-periodic behaviours, where a behaviour is periodic if it goes through a sequence of periods, i.e. of behaviours in which each active process executes one complete cycle, and a behaviour is pseudo-periodic, if it can be viewed as being composed of periods, possibly not happening in sequence. Thus, every periodic behaviour is pseudo-periodic. We identify a property, no overtaking, which a pseudo-periodic behaviour must have in order to be periodic.

This paper presents a survey of recent approaches in the application of the object-oriented paradigm to formal specification. The complexity of current information systems demands the use of a higher degree of formalism in the development process. Formal languages such as Z, VDM and Lotos have been used extensively in academic environments and research projects; however, their utilization in the ‘real world’ is still relatively small. The use of object-oriented concepts has now been suggested as a good solution to the lack of expressiveness that characterizes most of these languages. Several approaches addressing this issue have appeared in the literature recently, including extensions to most existing languages. In this paper we review some of these techniques and discuss relevant to the combination of formal methods and object orientation.

Abstract Pre-/postconditions have been extensively used in program specification, e.g. Z [Spi89], VDM [Jon86], and proof, e.g. Hoare logic, Dijkstra's guarded commands [DiF88]. In [ScP86, SPB90] the authors introduced neutral and central relations to formalise the concept of “the rest stays the same”. In this paper we abstract away from the specific definition of neutral relation given in [SPB90], through the mechanism of relational boolean algebras. This leads to the definition of implicitly central relations which are easier for the user in practical examples and facilitate the use of pre-/postcondition reasoning about truly concurrent behaviour.

The derivation of conformance tests for communication protocols is discussed. Protocol specifications are considered in the formal description technique LOTOS, which has been developed by the International Standards Organization. Test processes which preserve the structure of the protocol specifications are constructed. Laws are presented for handling basic LOTOS operators. The test processes obtained by applying these laws are related to the theoretical notion of canonical testers. The conversion of the test processes into finite test suites is discussed. Their relationship to current practice in test suite design is discussed.>