Ontologies have played an important role in the semantic web (or shortly "SW"). An ontology forms vocabularies and sentences, it expresses knowledge to be shared on the web. OWL was accepted by W3C as a language for representing a web ontology. Its core, OWL-DL, is essentially an XML encoding of an expressive Description Logic (DL) built upon Resource Description Framework (RDF) with a substantial fragment of RDF Schema (RDFS). The vocabularies defined in such an ontology consist of classes and properties; in logic classes can be treated as unary predicates, while properties as binary predicates, and all these predicates represent relations. OWL was successfully applied to SW in the past. However, some SW knowledge should be formulated more naturally in rules.
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