J. Ponte, and B. Croft. SIGIR '98: Proceedings of the 21st annual international ACM SIGIR conference on Research and development in information retrieval, page 275--281. New York, NY, USA, ACM Press, (1998)
H. Chieu, and H. Ng. Eighteenth national conference on Artificial intelligence, page 786--791. Menlo Park, CA, USA, American Association for Artificial Intelligence, (2002)