F. Kamareddine, M. Maarek, K. Retel, and J. Wells. Proceedings of the Sixth International Conference on MATHEMATICAL KNOWLEDGE MANAGEMENT, Lecture Notes in AI 4573, page 296-312. Springer, (2007)
F. Arzarello. the 24th Conference of the International Group for the Psychology of Mathematics Education (PME), 1, page 23--38. Hiroshima, Japan, (2000)