Molly Wilson and Alan Borning, <a href="http://www.cs.washington.edu/research/projects/weird/www/extending-hclp-naclp-89.html"> "Extending HCLP Languages: Nonmonotonicity and Inter-Hierarchy Comparison"</a>, Proceedings of the 1989 North American Logic Programming Conference, Cleveland, October 1989, pages 3-19.  <LI>
