1. libferris, http://witme.sourceforge.net/libferris.web/ , Visited Nov. 2005
2. Mail-sleuth homepage, http://www.mail-sleuth.com/ , Visited Jan. 2005
3. Aoki, P.M.: Implementation of extended indexes in POSTGRES. SIGIR Forum 25(1), 2–9 (1991), citeseer.ist.psu.edu/aoki91implementation.html
4. Blake, C., Merz, C.: UCI Repository of Machine Learning Databases. University of California, Irvine, CA, Department of Information and Computer Science (1998), http://www.ics.uci.edu/~mlearn/MLRepository.html
5. Lecture Notes in Artificial Intelligence;R. Cole,2001