Alan Borning, Richard Anderson, and Bjorn Freeman-Benson,  <a href="http://www.cs.washington.edu/research/projects/weird/www/indigo-uist-96.html"> "Indigo: A Local Propagation Algorithm for Inequality Constraints"</a>, <em>Proceedings of the 1996 ACM Symposium on User Interface Software and Technology</em>, pages 129-136.  A companion technical report, <a href="http://www.cs.washington.edu/research/projects/weird/www/indigo-tr.html"> "The Indigo Algorithm"</a>, UW tech report 96-05-01, contains additional material (proofs of theorems and definitions of interval operations).     </UL>
