1. Advanced Engineering and Sciences Division, ITT Industries, Inc., Alexandria, VA 22303-1410, USA. E-mail: iliano@itd.nrl.navy.mil
2. Sandia National Laboratories, Livermore, CA, USA. E-mail: nadurgi@sandia.gov
3. Computer Science Laboratory, SRI International, Menlo Park, CA 94025-3493, USA. E-mail: lincoln@csl.sri.com
4. Computer Science Department, Stanford University, Stanford, CA 94305-9045, USA. E-mail: jcm@cs.stanford.edu
5. Mathematics Department, University of Pennsylvania, Philadelphia, PA 19104-6395, USA. E-mail: scedrov@cis.upenn.edu