1. NEC Research Institute, 4 Independence Way, Princeton, NJ
2. Department of Computer Science, Institute of Mathematical and Physical Sciences, University of Tromso, N-9037 Tromso, Norway
3. IBM T.J. Watson Research Center, P.O. Box 218, Yorktown Heights, NY and Department of Computer Science, Yale University, P.O. Box 208285, New Haven, CT
4. Department of Computer Science, Princeton University, Princeton, NJ