1. Department of Electrical Engineering, Faculty of Engineering, Hiroshima University, Kagamiyama 1-4-1, Higashi-Hiroshima, 739, Japan
2. INRIA - SLOOP, 2004 Route des Lucioles, B.P 93, F-06902 Sophia-Antipolis, France
3. School of Computing Science, Simon Fraser University, Burnaby, British Columbia, V5A 1S6, Canada