Bernd Neumayr Katharina Grün Michael Schrefl Multi-Level Domain Modeling with M-Objects and M-Relationships. 107-116 2009 APCCM http://crpit.com/confpapers/CRPITV96Neumayr.pdf conf/apccm/2009 db/conf/apccm/apccm2009.html#NeumayrGS09