Molly Wilson and Alan Borning, <a href="http://www.cs.washington.edu/research/projects/weird/www/jlp-93.html"> "Hierarchical Constraint Logic Programming"</a>, The Journal of Logic Programming, special issue on Constraint Logic Programming, Vol. 16 Nos. 3 & 4, July-August 1993, pages 227-318.  <LI>
