1. IBM T. J. Watson Research Center, Hawthorne, NY 10532, USA
2. Institute for Computer Science-FORTH, Science and Technology Park, P.O. Box 1385, GR 711 10, Heraklion, Crete, Greece
3. Center For Telecomm. Research, Columbia University, NY 10027, USA
4. Department of Computer Science, Columbia University, NY 10027, USA