1. Department of Computer Science, Chungbuk National University, Cheongju, Chungbuk 360–763, Republic of Korea
2. Department of Computer Science, Korea Advanced Institute of Science and Technology, P.O. Box 150, Cheongryang, Seoul 130–650, Republic of Korea
3. Department of Computer Systems, University of Amsterdam, P.O. Box 41882, 1009 DB Amsterdam, The Netherlands