Department of Computer Science, Sogang University and Daumsoft Inc., Se-Ah Venture Tower, Seoul, Korea. shkim@lex.yonsei.ac.kr|Department of Computer Science, Sogang University and Daumsoft Inc., Se-Ah Venture Tower, Seoul, Korea. shkim@lex.yonsei.ac.kr|Department of Computer Science, Sogang University and Daumsoft Inc., Se-Ah Venture Tower, Seoul, Korea. shkim@lex.yonsei.ac.kr