Herman J. ter Horst Extending the RDFS Entailment Lemma. 77-91 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3298&spage=77 2004 conf/semweb/2004 International Semantic Web Conference db/conf/semweb/iswc2004.html#Horst04