1. Department of Computer Science and Engineering, Chung-Ang University, Seoul 156-756, Republic of Korea
2. Department of Computer Engineering and Mathematics, Rovira i Virgili University, 43007 Tarragona, Spain
3. Korea Institute of Science and Technology Information, Daejeon 305-806, Republic of Korea
4. School of Computing and Mathematics, Charles Sturt University, Bathurst, NSW 2795, Australia