1. Department of Computer Science, University of Rochester, Rochester, NY 14627, USA
2. Department Of Computer Education, Sunchon National University, Sunchon, Chonnam 540-742, Republic of Korea
3. Department Of Computer Science Education, Catholic University of Daegu, Gyeongsan, Gyeongbuk 712-702, Republic of Korea
4. Department of Information and Communication Engineering, Yeungnam University, Gyeongsan, Gyeongbuk 712-749, Republic of Korea