C. Schmitz, S. Staab, R. Studer, G. Stumme, and J. Tane. Proceedings of E-Learn 2002: World Conference on E-Learning in Corporate, Government, Healthcare, & Higher Education, October 15-19 2002, Montreal, Canada, page 909-915. (2002)
D. Fensel, S. Decker, M. Erdmann, and R. Studer. Proceedings of the Eleventh International Florida Artificial Intelligence Research Society Conference, page 131--135. AAAI Press, (1998)