F. Lehmann, and R. Wille. Conceptual structures: applications, implementation and theory, volume 954 of Lecture Notes in Artificial Intelligence, page 32-43. London, UK, Springer Verlag, (1995)
S. Krolak-Schwerdt, P. Orlik, and B. Ganter. Studies in Classification, Data Analysis, and Knowledge Organization, volume 4 of Information systems and data analysis, Springer, Berlin, (1994)