School of Computer Science and Engineering, Seoul National University, Seoul 151-742, Korea. kbhwang@bi.snu.ac.kr <kbhwang@bi.snu.ac.kr>|School of Computer Science and Engineering, Seoul National University, Seoul 151-742, Korea. kbhwang@bi.snu.ac.kr <kbhwang@bi.snu.ac.kr>|School of Computer Science and Engineering, Seoul National University, Seoul 151-742, Korea. kbhwang@bi.snu.ac.kr <kbhwang@bi.snu.ac.kr>|School of Computer Science and Engineering, Seoul National University, Seoul 151-742, Korea. kbhwang@bi.snu.ac.kr <kbhwang@bi.snu.ac.kr>