1. IBM Research Europe, The Hartree Centre, Daresbury WA4 4AD, U.K.
2. Department of Computer Science & Engineering, University of Connecticut, Storrs, Connecticut 06269, United States
3. IBM T. J. Watson Research, Cambridge, Massachusetts 02142, United States
4. The Hartree Centre, STFC Daresbury Laboratory, Warrington WA4 4AD, U.K.
5. IBM Research Europe, Dublin 15, Ireland
6. IBM Almaden Research Center, San Jose, California 95120, United States
7. Department of Computer Science, North Carolina State University, Raleigh, North Carolina 27695, United States