Gus Lopez, Bjorn Freeman-Benson, and Alan Borning, <a href="http://www.cs.washington.edu/research/projects/weird/www/kaleidoscope-ecoop-94.html"> "Constraints and Object Identity"</a>, Proceedings of the 1994 European Conference on Object-Oriented Programming, Bologna, Italy, July 1994, pages 260-279.  <LI>
