Marek Meyer Sonja Bergsträsser Birgit Zimmermann Christoph Rensing Ralf Steinmetz Modeling Modifications of Multimedia Learning Resources Using Ontology-Based Representations. 34-43 2007 conf/mmm/2007-1 MMM (1) http://dx.doi.org/10.1007/978-3-540-69423-6_4 db/conf/mmm/mmm2007-1.html#MeyerBZRS07