Swan Rocher

Graal: A Toolkit for Query Answering with Existential Rules

This paper presents Graal, a java toolkit dedicated to ontological query answering in the framework of existential rules. We consider knowledge bases composed of data and an ontology expressed by existential rules. The main features of Graal are the following: a basic layer that provides generic interfaces to store and query various kinds of data, forward chaining and query rewriting algorithms, structural analysis of decidability properties of a rule set, a textual format and its parser, and import of OWL 2 files. We describe in more detail the query rewriting algorithms, which rely on original techniques, and report some experiments.

Extending acyclicity notions for existential rules

Existential rules have been proposed for representing ontological knowledge, specifically in the context of Ontology-Based Query Answering. Entailment with existential rules is undecidable. We focus in this paper on conditions that ensure the termination of a breadth-first forward chaining algorithm known as the chase. First, we propose a new tool that allows to extend existing acyclicity conditions ensuring chase termination, while keeping good complexity properties. Second, we consider the extension to existential rules with nonmonotonic negation under stable model semantics and further extend acyclicity results obtained in the positive case.

Kiabora: an analyzer of existential rule bases

Kiabora is a software tool dedicated to the analysis of a base of existential rules (also called Datalog± rules). It is able to check if a set of rules belongs to a known class for which entailment is decidable, either directly, by checking some syntactic properties, or by means of its Graph of Rule Dependencies, which allows to combine decidable cases. Kiabora is available online via a simple web form. It is written in Java and is easily extensible to take new decidability results into account. It comes with a textual format, called DLGP (for Datalog Plus), which can be seen as an extension to usual plain Datalog format. In this paper, we briefly introduce the existential rule framework as well as decidability results and presents the analysis performed by Kiabora. More details are available on Kiabora website.

Inconsistency-Tolerant Query Answering: Rationality Properties and Computational Complexity Analysis

Generalising the state of the art, an inconsistency-tolerant semantics can be seen as a couple composed of a modifier operator and an inference strategy. In this paper we deepen the analysis of such general setting and focus on two aspects. First, we investigate the rationality properties of such semantics for existential rule knowledge bases. Second, we unfold the broad landscape of complexity results of inconsistency-tolerant semantics under a specific (yet expressive) subclass of existential rules.

Bringing existential variables in answer set programming and bringing non-monotony in existential rules: two sides of the same coin

This article deals with the combination of ontologies and rules by means of existential rules and answer set programming. Existential rules have been proposed for representing ontological knowledge, specifically in the context of Ontology- Based Data Access. Furthermore Answer Set Programming (ASP) is an appropriate formalism to represent various problems issued from Artificial Intelligence and arising when available information is incomplete. The combination of the two formalisms requires to extend existential rules with nonmonotonic negation and to extend ASP with existential variables. In this article, we present the syntax and semantics of Existential Non Monotonic Rules (ENM-rules) using skolemization which join together the two frameworks. We formalize its links with standard ASP. Moreover, since entailment with existential rules is undecidable, we present conditions that ensure the termination of a breadth-first forward chaining algorithm known as the chase and we discuss extension of these results in the nonmonotonic case.

\exists -ASP for Computing Repairs with Existential Ontologies

Repair-based techniques are a standard way of dealing with inconsistency in the context of ontology-based data access where several inconsistency-tolerant semantics have been mainly proposed for lightweight description logics. In this paper we present a generic transformation from knowledge bases expressed within existential rules formalism into an ASP program. We propose different strategies for this transformation, and highlight the ones for which answer sets of the generated program correspond to various kinds of repairs used in inconsistency-tolerant inferences.

On the Explanation of SameAs Statements using Argumentation

Due to the impressive growing of the LOD graph in the last years, assuring the quality of its content is becoming a very important issue. Thus, it is crucial to design techniques for supporting experts in validating facts and links in complex data sources. Here, we focus on identity links (sameAs) and apply argumentation semantics to (i) detect inconsistencies in sameAs statements and to (ii) explain them to the experts using dialogues. We formalize the framework, explaining its purposes. Finally we provide a promising preliminary evaluation and discuss on some interesting future directions we foresee.