Alan Borning, Michael Maher, Amy Martindale, and Molly Wilson, <a href="http://www.cs.washington.edu/research/projects/weird/www/hclp-iclp-89.html"> "Constraint Hierarchies and Logic Programming"</a>, Proceedings of the Sixth International Logic Programming Conference, Lisbon, Portugal, June 1989, pages 149-164.  <LI>
