Bjorn Freeman-Benson and Alan Borning, <a href="http://www.cs.washington.edu/research/projects/weird/www/kaleidoscope-ecoop-92.html"> "Integrating Constraints with an Object-Oriented Language"</a>, Proceedings of the 1992 European Conference on Object-Oriented Programming, June 1992, pages 268-286.  <LI>
