G. Mineau, G. Stumme, and R. Wille. Conceptual Structures: Standards and Practices. Proc. ICCS '99, volume 1640 of LNAI, page 423-441. Heidelberg, Springer, (1999)
R. Kondor, and J. Lafferty. ICML '02: Proceedings of the Nineteenth International Conference on Machine Learning, page 315--322. San Francisco, CA, USA, Morgan Kaufmann Publishers Inc., (2002)