S. Strahringer, R. Wille, and U. Wille. Proceedings of the 9th International Conference on Conceptual Structures (ICCS 2001), volume 2120 of Lecture Notes in Computer Science, page 169-186. Springer, (2001)
P. Hitzler, and G. Zhang. Proceedings of the 12th International Conference on Conceptual Structures (ICCS 2004), volume 3127 of Lecture Notes in Computer Science, page 170-185. Springer, (2004)