Maria Bergholtz Paul Johannesson Classifying the Semantics of Relationships in Conceptual Modeling by Categorization of Roles. 199-203 2001 conf/nldb/2001 NLDB db/conf/nldb/nldb2001.html#BergholtzJ01 http://subs.emis.de/LNI/Proceedings/Proceedings03/article61.html