J. Ontrup, and H. Ritter. Proceedings of PKDD-01, 5th European Conference on Principles and Practice of Knowledge Discovery in Databases, page 338--349. Springer, LNAI 2168, (2001)
J. Suzuki, T. Hirao, Y. Sasaki, and E. Maeda. ACL '03: Proceedings of the 41st Annual Meeting on Association for Computational Linguistics, page 32--39. Morristown, NJ, USA, Association for Computational Linguistics, (2003)