Munish Gandhi, Edward L. Robertson, Dirk Van Gucht. Levelled Entity Relationship Model. In Pericles Loucopoulos, editor, Entity-Relationship Approach - ER 94, Business Modelling and Re-Engineering, 13th International Conference on the Entity-Relationship Approach, Manchester, U.K., December 13-16, 1994, Proceedings. Volume 881 of Lecture Notes in Computer Science, pages 420-436, Springer, 1994. [doi]