1. The DLV home page, http://www.dbai.tuwien.ac.at/proj/dlv/
2. The ECLiPSe website at Imperial College, http://www-icparc.doc.ic.ac.uk/eclipse/
3. The SMODELS home page, http://www.tcs.hut.fi/Software/smodels/
4. The XSB home page, http://www.cs.sunysb.edu/~sbprolog/xsb-page.html
5. Lecture Notes in Computer Science;R. Barbuti,1987