. We proposed a syntactical format, the path format, for structured operational semantics in which predicates may occur. We proved that strong bisimulation is a congruence for all the operators that can be defined within the path format. To show that this format is useful we provided many examples that we took from the literature about CCS, CSP, and ACP; they do satisfy the path format but no formats proposed by others. The examples include concepts like termination, convergence, divergence, weak bisimulation, a zero object, side conditions, functions, real time, discrete time, sequencing, negative premises, negative conclusions, and priorities (or a combination of these notions). Key Words & Phrases: structured operational semantics, term deduction system, transition system specification, structured state system, labelled transition system, strong bisimulation, congruence theorem, predicate. 1980 Mathematics Subject Classification (1985 Revision): 68Q05, 68Q55. CR Categories: D.3.1...

Many process algebras are defined by structural operational semantics (SOS). Indeed, most such definitions are nicely structured and fit the GSOS format of [15]. We give a procedure for converting any GSOS language definition to a finite complete equational axiom system (possibly with one infinitary induction principle) which precisely characterizes strong bisimulation of processes.

Concurrent ML (CML) is an extension of Standard ML of New Jersey with concurrent features similar to those of process algebra. In this paper, we build upon John Reppy's reduction semantics for CML by constructing a compositional operational semantics for a fragment of CML, basedon higherorder process algebra. Using the operational semantics we generalise the notion of weak bisimulation equivalence to build a semantic theory of CML. We give some small examples of proofs about CML expressions, and show that our semantics corresponds to Reppy's up to weak first-order bisimulation. 1 Introduction There have been various attempts to extend standard programming languages with concurrent or distributed features, (Giacalone et al., 1989; Holmstrom, 1983; Nikhil, 1990). Concurrent ML (CML) (Reppy, 1991a; Reppy, 1992; Panangaden & Reppy, 1996) is a practical and elegant example. The language Standard ML is extended with two new type constructors, one for generating communication channels, and t...

This paper combines and extends the material of [GG-a/c/d/e], except for the part in [GG-c] on refinement of transitions in Petri nets and the discussion of TCSP-like parallel composition in [GG-e]. An informal presentation of some basic ingredients of this paper appeared as [GG-b]. Among others, the treatment of action refinement in stable and non-stable event structures is new. The research reported here was supported by Esprit project 432 (METEOR), Esprit Basic Research Action 3148 (DEMON), Sonderforschungsbereich 342 of the TU Munchen, ONR grant N00014-92-J-1974 and the Human Capital and Mobility Cooperation Network EXPRESS (Expressiveness of Languages for Concurrency). Contents

The expressive power of process algebras is investigated in a general setting of structural operational semantics. The notion of an effective operational semantics is introduced and it is observed that no effective operational semantics for an enumerable language can specify all effective process graphs up to trace equivalence. A natural class of Plotkin style SOS specifications is identified, containing the guarded versions of calculi like CCS, SCCS, Meije and ACP, and it is proved that any specification in this class induces an effective operational semantics. Using techniques introduced by Bloom, it is shown that for the guarded versions of CCS-like calculi, there is a double exponential bound on the speed with which the number of outgoing transitions in a state can grow. As a corollary of this result it follows that two expressiveness results of De Simone for Meije and SCCS depend in a fundamental way on the use of unguarded recursion. A final result of this paper is that all operators definable via a finite number of rules in a format due to De Simone, are derived operators in the simple process calculus PC. 1991 Mathematics Subject Classification: 68Q05, 68Q10, 68Q55, 68Q75, 03D20. 1991 CR Categories: D.3.1, D.3.3, F.1.1, F.1.2, F.3.2, F.4.1. Keywords & Phrases: process algebra, PC, labeled transition systems, process graphs, effective process graphs, effective operational semantics, structural operational semantics, expressiveness, bisimulation equivalence, trace equivalence, action transducers. Notes: Most of this work was carried out while the author was at the MIT Laboratory for Computer Science, supported by ONR contract N00014-85-K-0168. Part of this work took place in the context of the ESPRIT Basic Research Action 7166, CONCUR2. This p...

We consider a class of Kripke Structures in which the atomic propositions are events. This enables us to represent worlds as sets of events and the transition and satisfaction relations of Kripke structures as the subset and membership relations on sets. We use this class, called event Kripke structures, to model concurrency. The obvious semantics for these structures is a true concurrency semantics. We show how several aspects of concurrency can be easily defined, and in addition get distinctions between causality and enabling, and choice and nondeterminism. We define a duality for event Kripke structures, and show how this duality enables us to convert between imperative and declarative views of programs, by treating states and events on the same footing. We provide pictorial representations of both these views, each encoding all the information to convert to the other. We define a process algebra of event Kripke structures, showing how to combine them in the usual ways---parallel co...

A simple untyped language based on CML, Concurrent ML, is defined and analysed. The language contains a spawn operator for initiating new independent threads of computation and constructs for the exchange of data between these threads. A denotational model for the language is presented where denotations correspond to computations of values rather than simply values. It is shown to be fully abstract with respect to a behavioural preorder based on contextual testing. 1 Introduction The language Concurrent ML (CML), [18], is one of a number of recent languages which seeks to combine aspects of functional and concurrent programming. Standard ML, [19], is augmented with the ability to spawn off new independent threads of computation. Further constructs are added to enable these threads to synchronise and exchange data on communication channels. As it includes higherorder objects, which can be exchanged between threads as data, new channel name generation, and the ability to form abstractio...

Process algebras are a frequently used tool for the specification and verification of distributed reactive systems. In process algebras, actions are used to denote the basic entities of systems. In general, actions are just abstract names with no particular interpretation. The semantics of a system description, given in form of a process algebra term, is not influenced by the choice of names.

A many-sorted variant, called stalk format, of the single sorted tyft-format for transition system specifications, introduced by Groote and Vaandrager, is proposed. The stalk format is shown to be a convenient formalism to express continuation-style transition systems for languages incorporating concepts as, e.g., process creation or backtracking, for which the existing formats seem less adequate. It is proved, extending a similar result for the single sorted case, that for an appropriate generalization of (strong) bisimilarity for the present many-sorted setting, bisimulation with respect to a transition system specification in stalk format, is a congruence. It is argued that the several conditions, required for the type of transition system specification put forward in the paper, can not be relaxed without loosing this congruence. Finally, the present format is compared with several existing ones in the literature, viz. De Simone-, GSOS- and pure tyft-format. Keywords and phrases se...

. In this paper we study a special operator for sequential composition, which is defined relative to a dependency relation over the actions of a given system. The idea is that actions which are not dependent (intuitively because they share no common resources) do not have to wait for one another to proceed, even if they are composed sequentially. Such a notion has been studied before in a linear-time setting, but until recently there has been no systematic investigation in the context of process algebras. We give a structural operational semantics for a process algebraic language containing such a sequential composition operator, which shows some interesting interplay with choice. We give a complete axiomatisation of strong bisimilarity and we show consistency of the operational semantics with an event-based denotational semantics developed recently by the second author. The axiom system allows to derive the communication closed layers law, which in the linear time setting has been sho...