1,270 research outputs found
Priorities Without Priorities: Representing Preemption in Psi-Calculi
Psi-calculi is a parametric framework for extensions of the pi-calculus with
data terms and arbitrary logics. In this framework there is no direct way to
represent action priorities, where an action can execute only if all other
enabled actions have lower priority. We here demonstrate that the psi-calculi
parameters can be chosen such that the effect of action priorities can be
encoded.
To accomplish this we define an extension of psi-calculi with action
priorities, and show that for every calculus in the extended framework there is
a corresponding ordinary psi-calculus, without priorities, and a translation
between them that satisfies strong operational correspondence. This is a
significantly stronger result than for most encodings between process calculi
in the literature.
We also formally prove in Nominal Isabelle that the standard congruence and
structural laws about strong bisimulation hold in psi-calculi extended with
priorities.Comment: In Proceedings EXPRESS/SOS 2014, arXiv:1408.127
Defeasible Reasoning in SROEL: from Rational Entailment to Rational Closure
In this work we study a rational extension of the low complexity
description logic SROEL, which underlies the OWL EL ontology language. The
extension involves a typicality operator T, whose semantics is based on Lehmann
and Magidor's ranked models and allows for the definition of defeasible
inclusions. We consider both rational entailment and minimal entailment. We
show that deciding instance checking under minimal entailment is in general
-hard, while, under rational entailment, instance checking can be
computed in polynomial time. We develop a Datalog calculus for instance
checking under rational entailment and exploit it, with stratified negation,
for computing the rational closure of simple KBs in polynomial time.Comment: Accepted for publication on Fundamenta Informatica
Primitives for Contract-based Synchronization
We investigate how contracts can be used to regulate the interaction between
processes. To do that, we study a variant of the concurrent constraints
calculus presented in [1], featuring primitives for multi-party synchronization
via contracts. We proceed in two directions. First, we exploit our primitives
to model some contract-based interactions. Then, we discuss how several models
for concurrency can be expressed through our primitives. In particular, we
encode the pi-calculus and graph rewriting.Comment: In Proceedings ICE 2010, arXiv:1010.530
Reasoning about exceptions in ontologies: from the lexicographic closure to the skeptical closure
Reasoning about exceptions in ontologies is nowadays one of the challenges
the description logics community is facing. The paper describes a preferential
approach for dealing with exceptions in Description Logics, based on the
rational closure. The rational closure has the merit of providing a simple and
efficient approach for reasoning with exceptions, but it does not allow
independent handling of the inheritance of different defeasible properties of
concepts. In this work we outline a possible solution to this problem by
introducing a variant of the lexicographical closure, that we call skeptical
closure, which requires to construct a single base. We develop a bi-preference
semantics semantics for defining a characterization of the skeptical closure
Default reasoning using maximum entropy and variable strength defaults
PhDThe thesis presents a computational model for reasoning with partial information
which uses default rules or information about what normally happens. The idea is
to provide a means of filling the gaps in an incomplete world view with the most
plausible assumptions while allowing for the retraction of conclusions should they
subsequently turn out to be incorrect. The model can be used both to reason from
a given knowledge base of default rules, and to aid in the construction of such
knowledge bases by allowing their designer to compare the consequences of his
design with his own default assumptions. The conclusions supported by the proposed
model are justified by the use of a probabilistic semantics for default rules
in conjunction with the application of a rational means of inference from incomplete
knowledge the principle of maximum entropy (ME). The thesis develops
both the theory and algorithms for the ME approach and argues that it should be
considered as a general theory of default reasoning.
The argument supporting the thesis has two main threads. Firstly, the ME approach
is tested on the benchmark examples required of nonmonotonic behaviour,
and it is found to handle them appropriately. Moreover, these patterns of commonsense
reasoning emerge as consequences of the chosen semantics rather than
being design features. It is argued that this makes the ME approach more objective,
and its conclusions more justifiable, than other default systems. Secondly, the
ME approach is compared with two existing systems: the lexicographic approach
(LEX) and system Z+. It is shown that the former can be equated with ME under
suitable conditions making it strictly less expressive, while the latter is too crude to
perform the subtle resolution of default conflict which the ME approach allows. Finally,
a program called DRS is described which implements all systems discussed
in the thesis and provides a tool for testing their behaviours.Engineering and Physical Science Research Council (EPSRC
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