Jianhua Zhu, Rodolphe Nassif, Pankaj Goyal, Pamela Drew, Bertil Askelid. Incorporating a Model Hierarchy into the ER Paradigm. In Hannu Kangassalo, editor, Proceedings of the 9th International Conference on Entity-Relationship Approach (ER 90), 8-10 October, 1990, Lausanne, Switzerland. pages 67-80, ER Institute, 1990. [doi]