1. AI Section, Electronics and Telecommunications Research Institute, 161 Kajong-Dong, Yusong-Gu, Taejon 305-350, Korea
2. Computer Systems Lab., Computer Science Department, Korea Advanced Institute of Science and Technology, Yusong P.O. Box 111, Taejon 305-600, Korea
3. Department of Electrical and Computer Engineering, University of California, ET544E Irvine, CA 92697, USA