1. Department of Mathematics Education, Sungkyunkwan University, Seoul, South Korea
2. Center for Geometry and Physics, Institute for Basic Science, Pohang, South Korea; and Department of Mathematics, Chungbuk National University, Cheongju, South Korea
3. Osaka City University Advanced Mathematics Institute (OCAMI), and Department of Mathematics, Graduate School of Science, Osaka City University, Osaka, Japan
4. Department of Mathematics Education, Jeonju University, Jeonju, South Korea