Abstract. This is a system description of the Description Logic reasoner FaCT++. The reasoner implements a tableaux decision procedure for the well known SHOIQ description logic, with additional support for datatypes, including strings and integers. The system employs a wide range of performance enhancing optimisations, including both standard techniques (such as absorption and model merging) and newly developed ones (such as ordering heuristics and taxonomic classification). FaCT++ can, via the standard DIG interface, be used to provide reasoning services for ontology engineering tools supporting the OWL DL ontology language.

Although the OWL Web Ontology Language adds considerable expressive power to the Semantic Web it does have expressive limitations, particularly with respect to what can be said about properties. We present SWRL (the Semantic Web Rules Language), a Horn clause rules extension to OWL that overcomes many of these limitations. SWRL extends OWL in a syntactically and semantically coherent manner: the basic syntax for SWRL rules is an extension of the abstract syntax for OWL DL and OWL Lite; SWRL rules are given formal meaning via an extension of the OWL DL model-theoretic semantics; SWRL rules are given an XML syntax based on the OWL XML presentation syntax; and a mapping from SWRL rules to RDF graphs is given based on the OWL RDF/XML exchange syntax. We discuss the expressive power of SWRL, showing that the ontology consistency problem is undecidable, provide several examples of SWRL usage, and discuss a prototype implementation of reasoning support for SWRL.

Tableau algorithms are currently the most widely used and empirically the fastest algorithms for reasoning in expressive description logics, including the important description logics SHIQ and SHOIQ. Achieving a high level of performance on terminological reasoning in expressive description logics when using tableaubased algorithms requires the incorporation of a wide variety of optimizations. The description logic system FaCT++ implements a wide variety of such optimizations, some present in other reasoners and some novel or refined in FaCT++.

Although the OWL Web Ontology Language adds considerable expressive power to the Semantic Web it does have expressive limitations, particularly with respect to what can be said about properties. We present SWRL (the Semantic Web Rules Language), a Horn clause rules extension to OWL that overcomes many of these limitations. SWRL extends OWL in a syntactically and semantically coherent manner: the basic syntax for SWRL rules is an extension of the abstract syntax for OWL DL and OWL Lite; SWRL rules are given formal meaning via an extension of the OWL DL model-theoretic semantics; SWRL rules are given an XML syntax based on the OWL XML presentation syntax; and a mapping from SWRL rules to RDF graphs is given based on the OWL RDF/XML exchange syntax. We discuss the expressive power of SWRL, showing that the ontology consistency problem is undecidable, provide several examples of SWRL usage, and discuss a prototype implementation of reasoning support for SWRL.

Abstract. OWL is an ontology language developed by the W3C, and although initially developed for the Semantic Web, OWL has rapidly become a de facto standard for ontology development in general. The design of OWL was heavily influenced by research in description logics, and the specification includes a formal semantics. One of the goals of this formal approach was to provide interoperability: different OWL reasoners should provide the same results when processing the same ontologies. In this paper we present a system that allows users: (a) to test and compare OWL reasoners using an extensible library of real-life ontologies; (b) to check the "correctness" of the reasoners by comparing the computed class hierarchy; (c) to compare the performance of the reasoners when performing this task; and (d) to use SQL queries to analyse and present the results in any way they see fit.

Abstract. OWL DL corresponds to a Description Logic (DL) that is a fragment of classical first-order predicate logic (FOL). Therefore, the standard methods of automated reasoning for full FOL can potentially be used instead of dedicated DL reasoners to solve OWL DL reasoning tasks. In this paper we report on some experiments designed to explore the feasibility of using existing general-purpose FOL provers to reason with OWL DL. We also extend our approach to SWRL, a proposed rule language extension to OWL.

Abstract. For ontology reuse and integration, a number of approaches have been devised that aim at identifying modules, i.e., suitably small sets of "relevant" axioms from ontologies. Here we consider three logically sound notions of modules: MEX modules, only applicable to inexpressive ontologies; modules based on semantic locality, a sound approximation of the first; and modules based on syntactic locality, a sound approximation of the second (and thus the first), widely used since these modules can be extracted from SROIQ ontologies in time polynomial in the size of the ontology. In this paper we investigate the quality of both approximations over a large corpus of ontologies. In particular, we show with statistical significance that, in most cases, there is no difference between the two module notions based on locality; where they differ, the additional axioms are in general unproblematic since either they can be easily ruled out or their number is relatively small. Finally, we show that the same can be said about the relation between MEX and locality-based modules.

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