Ramez Elmasri James A. Weeldreyer Alan R. Hevner The Category Concept: An Extension to the Entity-Relationship Model. 75-116 1985 1 1 Data Knowl. Eng. db/journals/dke/dke1.html#ElmasriWH85 http://dx.doi.org/10.1016/0169-023X(85)90027-8