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Nominal Unification
 Theoretical Computer Science
, 2003
"... We present a generalisation of firstorder unification to the practically important case of equations between terms involving binding operations. A substitution of terms for variables solves such an equation if it makes the equated terms #equivalent, i.e. equal up to renaming bound names. For the a ..."
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Cited by 53 (20 self)
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We present a generalisation of firstorder unification to the practically important case of equations between terms involving binding operations. A substitution of terms for variables solves such an equation if it makes the equated terms #equivalent, i.e. equal up to renaming bound names. For the applications we have in mind, we must consider the simple, textual form of substitution in which names occurring in terms may be captured within the scope of binders upon substitution. We are able to take a `nominal' approach to binding in which bound entities are explicitly named (rather than using nameless, de Bruijnstyle representations) and yet get a version of this form of substitution that respects #equivalence and possesses good algorithmic properties. We achieve this by adapting an existing idea and introducing a key new idea. The existing idea is terms involving explicit substitutions of names for names, except that here we only use explicit permutations (bijective substitutions). The key new idea is that the unification algorithm should solve not only equational problems, but also problems about the freshness of names for terms. There is a simple generalisation of the classical firstorder unification algorithm to this setting which retains the latter's pleasant properties: unification problems involving #equivalence and freshness are decidable; and solvable problems possess most general solutions.
Model checking for nominal calculi
 IN FOSSACS, VOLUME 3441 OF LNCS
, 2005
"... Nominal calculi have been shown very effective to formally model a variety of computational phenomena. The models of nominal calculi have often infinite states, thus making model checking a difficult task. In this note we survey some of the approaches for model checking nominal calculi. Then, we f ..."
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Cited by 6 (2 self)
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Nominal calculi have been shown very effective to formally model a variety of computational phenomena. The models of nominal calculi have often infinite states, thus making model checking a difficult task. In this note we survey some of the approaches for model checking nominal calculi. Then, we focus on HistoryDependent automata, a syntaxfree automatonbased model of mobility. HistoryDependent automata have provided the formal basis to design and implement some existing verification toolkits. We then introduce a novel syntaxfree setting to model the symbolic semantics of a nominal calculus. Our approach relies on the notions of reactive systems and observed borrowed contexts introduced by Leifer and Milner, and further developed by Sassone, Lack and Sobocinski. We argue that the symbolic semantics model based on borrowed contexts can be conveniently applied to web service discovery and binding.
Spatial Information Distribution in Constraintbased
"... Abstract. We introduce spatial and epistemic process calculi for reasoning about spatial information and knowledge distributed among the agents of a system. We introduce domaintheoretical structures to represent spatial and epistemic information. We provide operational and denotational techniques f ..."
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Abstract. We introduce spatial and epistemic process calculi for reasoning about spatial information and knowledge distributed among the agents of a system. We introduce domaintheoretical structures to represent spatial and epistemic information. We provide operational and denotational techniques for reasoning about the potentially infinite behaviour of spatial and epistemic processes. We also give compact representations of infinite objects that can be used by processes to simulate announcements of common knowledge and global information. Introduction. Distributed systems have changed substantially in the recent past with the advent of phenomena like social networks and cloud computing. In the previous incarnation of distributed computing [16] the emphasis was on consistency, faulttolerance, resource management and related topics; these were all characterized by interaction between processes. Research proceeded along two lines: the algorithmic side which
Namespace logic: A logic for a reflective higherorder calculus
"... Abstract. In [19] it was observed that a theory like the πcalculus, dependent on a theory of names, can be closed, through a mechanism of quoting, so that (quoted) processes provide the necessary notion of names. Here we expand on this theme by examining a construction for a HennessyMilner logic c ..."
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Abstract. In [19] it was observed that a theory like the πcalculus, dependent on a theory of names, can be closed, through a mechanism of quoting, so that (quoted) processes provide the necessary notion of names. Here we expand on this theme by examining a construction for a HennessyMilner logic corresponding to an asynchronous messagepassing calculus built on a notion of quoting. Like standard HennessyMilner logics, the logic exhibits formulae corresponding to sets of processes, but a new class of formulae, corresponding to sets of names, also emerges. This feature provides for a number of interesting possible applications from security to data manipulation. Specifically, we illustrate formulae for controlling process response on ranges of names reminiscent of a (static) constraint on port access in a firewall configuration. Likewise, we exhibit formulae in a namesasdata paradigm corresponding to validation for fragment of XML Schema. 1
Policybased Coordination in PAGODA:
"... PAGODA (Policy And GOal Based Distributed Autonomy) is a modular architecture for specifying and prototyping autonomous systems. A PAGODA node (agent) interacts with its environment by sensing and affecting, driven by goals to achieve and constrained by policies. A PAGODA system is a collection of P ..."
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PAGODA (Policy And GOal Based Distributed Autonomy) is a modular architecture for specifying and prototyping autonomous systems. A PAGODA node (agent) interacts with its environment by sensing and affecting, driven by goals to achieve and constrained by policies. A PAGODA system is a collection of PAGODA nodes cooperating to achieve some mutual goal. This paper describes a specification of PAGODA using the Russian Dolls model of policybased coordination. In PAGODA there are two forms of coordination: local and global. Local coordination is used to compose the components of a PAGODA node. The local coordinator is concerned with ensuring component level synchronization constraints, cross component message ordering constraints, routing of notifications, and interaction with the external world. The global coordinator is concerned with dissemination of information, negotiation of responsibilities, and synchronization of activities. Requirements for a PAGODA node coordinator are given and an example set of policies is specified. Principles for showing that the policies satisfy the requirements are discussed as a first step toward a logic of policybased coordination. Development of a distributed coordinator is the subject of ongoing work. Some challenges and possible solutions are discussed.