Jelena Jovanovic Dragan Gasevic Katrien Verbert Erik Duval Ontology of Learning Object Content Structure. 322-329 2005 AIED conf/aied/2005 db/conf/aied/aied2005.html#JovanovicGVD05 http://www.booksonline.iospress.nl/Content/View.aspx?piid=1325