T. Amghar, T. Charnois, and D. Battistelli. Proceedings of the 9th International Conference on Conceptual Structures (ICCS 2001), volume 2120 of Lecture Notes in Computer Science, page 29-43. Springer, (2001)
H. Schärfe, and P. Øhrstrøm. Proceedings of the 11th International Conference on Conceptual Structures (ICCS 2003), volume 2746 of Lecture Notes in Computer Science, page 201-214. Springer, (2003)