D. Spikol, and J. Eliasson. The 6th IEEE International Conference on Wireless, Mobile, and Ubiquitous Technologies in Education, page 137--141. IEEE, (2010)
M. Mariotti. Proceedings of the 22nd Conference of the International Group for the Psychology of Mathematics Education, 1, page 180--195. Stellenbosch, South Africa, PME, University of Stellenbosch, (July 1998)