A simple efficiency preorder for CCS processes is introduced, in which p ! q means that q is at least as fast as p, or more generally, p uses at least as much resources as q. It is shown to be preserved by all CCS contexts except summation and it is used to analyse a non-trivial example: different implementations of a bounded buffer. Finally a sound and complete proof system for finite processes is given. This paper appeared earlier in [1] and [2]. This version of the paper has been revised, corrected and extended to include more operators, more examples and some additional remarks and observations by the first author. Hence all errors are entirely his responsiblity. Most of this work was done while the first author was at the University of Sussex and supported by SERC grant GR/D 97368 of the Science and Engineering Research Council of Great Britain. y The second author would like to acknowledge the support of ESPIRIT II. 1 Introduction A large number of behavioural equivale...

We study a variant of Levi and Sangiorgi's Safe Ambients (SA) enriched with passwords (SAP). In SAP by managing passwords, for example generating new ones and distributing them selectively, an ambient may now program who may migrate into its computation space, and when. Moreover in SAP an ambient may provide different services depending on the passwords exhibited by its incoming clients. We give an lts based operational semantics for SAP and a labelled bisimulation based equivalence which is proved to coincide with barbed congruence. Our notion of bisimulation is used to prove a set of algebraic laws which are subsequently exploited to prove more significant examples. 1

We propose an object-oriented calculus with internal concurrency and class-based inheritance that is built upon the join calculus. Method calls, locks, and states are handled in a uniform manner, using asynchronous messages. Classes are partial message definitions that can be combined and transformed. We design operators for behavioral and synchronization inheritance. We also give a type system that statically enforces basic safety properties. Our model is compatible with the JoCaml implementation

communication primitives acting across ambient boundaries. Expressiveness is achieved at the price of communication interferences on message reception whose resolution requires synchronisation of activities at multiple, distributed locations. We study a variant of BA aimed at controlling communication interferences as well as mobility ones. Our calculus draws inspiration from Safe Ambients (SA) (with passwords) and modifies the communication mechanism of BA. Expressiveness is maintained through a new form of co-capability that at the same time registers incoming agents with the receiver ambient and performs access control.

. We precisely characterize a class of cyclic lambda-graphs, and then give a sound and complete axiomatization of the terms that represent a given graph. The equational axiom system is an extension of lambda calculus with the letrec construct. In contrast to current theories, which impose restrictions on where the rewriting can take place, our theory is very liberal, e.g., it allows rewriting under lambda-abstractions and on cycles. As shown previously, the reduction theory is non-confluent. We thus introduce an approximate notion of confluence. Using this notion we define the infinite normal form or L'evy-Longo tree of a cyclic term. We show that the infinite normal form defines a congruence on the set of terms. We relate our cyclic lambda calculus to the traditional lambda calculus and to the infinitary lambda calculus. Since most implementations of non-strict functional languages rely on sharing to avoid repeating computations, we develop a variant of our calculus that enforces the ...

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We study the behavioural theory of Cardelli and Gordon's Mobile Ambients. We give an LTS based operational semantics, and a labelled bisimulation based equivalence that coincides with reduction barbed congruence. We also provide two up-to proof techniques that we use to prove a set of algebraic laws, including the perfect firewall equation.

In a previous paper we had dened the notion of an eciency preorder for concurrent systems. In this paper, we present a coarser relation, called the elaboration preorder, which is ner than observational equivalence. Further, this preorder is incomparable with the almostweak bismulation preorder of Milner and Sangiorgi. In particular, the elaboration preorder is preserved under all contexts except summation. The largest precongruence contained in it, which we call conformance, is obtained by the usual means and a complete axiomatization for conformance of nite processes is given. The paper ends with an example to show the use of this relation. 1 Introduction In [1] the eciency preorder was dened on processes and a proof system was given and shown to be complete for nite processes. It was shown that it was possible to compare eciencies of dierent implementations of the same specication with little extra eort than that required to prove their correctness. However, the eciency preo...

An improvement theory is a variant of the standard theories of observational approximation (or equivalence) in which the basic observations made of a functional program's execution include some intensionalinformation about, for example, the program's computational cost. One program is an improvement of another if its execution is more efficient in any program context. In this article we give an overview of our work on the theory and applications of improvement. Applications include reasoning about time properties of functional programs, and proving the correctness of program transformation methods. We also introduce a new application, in the form of some bisimulationlike proof techniques for equivalence, with something of the flavour of Sangiorgi's "bisimulation up-to expansion and context".

We develop an operational theory of higher-order functions, recursion, and fair non-determinism for a non-trivial, higher-order, call-by-name functional programming language extended with McCarthy's amb. Implemented via fair parallel evaluation, functional programming with amb is very expressive. However, conventional semantic fixed point principles for reasoning about recursion fail in the presence of fairness. Instead, we adapt higher-order operational methods to deal with fair non-determinism. We present two natural semantics, describing mayand must-convergence, and define a notion of contextual equivalence over these two modalities. The presence of amb raises special difficulties when reasoning about contextual equivalence. In particular, we report on a challenging open problem with regard to the validity of bisimulation proof methods. We develop two sound and useful reasoning methods which, in combination, enable us to prove a rich collection of laws for contextual...