J. Sowa. Proceedings of the Linguistic on Conceptual Structures: Logical Linguistic, and Computational Issues, 1867, page 55 - 81. Springer-Verlag London, UK, (2000)
M. Balabanović, L. Chu, and G. Wolff. CHI '00: Proceedings of the SIGCHI conference on Human factors in computing systems, page 564--571. New York, NY, USA, ACM, (2000)