M. Strube, and S. Ponzetto. Proceedings of the National Conference on Artificial Intelligence, 21, page 1419. Menlo Park, CA; Cambridge, MA; London; AAAI Press; MIT Press; 1999, (2006)
L. Dietz, S. Bickel, and T. Scheffer. Proceedings of the 24th international conference on Machine learning, page 233--240. New York, NY, USA, ACM, (2007)