S. Kuznetsov, S. Obiedkov, and C. Roth. Proceedings of the 15th International Conference on Conceptual Structures (ICCS 2007), volume 4604 of Lecture Notes in Artificial Intelligence, page 241-254. Berlin, Heidelberg, Springer-Verlag, (July 2007)
F. Lehmann, and R. Wille. Conceptual structures: applications, implementation and theory, 954, page 32-43. Berlin--Heidelberg--New~York, Springer--Verlag, (1995)