Boris Motik; Peter F. Patel-Schneider; Bernardo Cuenca Grau. <a href="http://www.w3.org/TR/2009/REC-owl2-direct-semantics-20091027/"><cite>OWL 2 Web Ontology Language:Direct Semantics.</cite></a> 27 October 2009. W3C Recommendation. URL: <a href="http://www.w3.org/TR/2009/REC-owl2-direct-semantics-20091027/">http://www.w3.org/TR/2009/REC-owl2-direct-semantics-20091027/</a>