1. Sandia National Labs, P.O. Box 969, Livermore, CA 94551, USA. Tel.: ; Fax: ; E-mail: nadurgi@sandia.gov
2. Computer Science Department, Stanford University, Stanford, CA 94305, USA. E-mail: jcm@cs.stanford.edu
3. Kestrel Institute, Palo Alto, CA 94304, USA. E-mail: dusko@kestrel.edu