S. Bereg, M. Völker, A. Wolff, and Y. Zhang. Proc. 33rd Int. Conf. Current Trends Theory &
Practice Comput. Sci. (SOFSEM'07), volume 4362 of Lecture Notes in Computer Science, page 177--186. Springer-Verlag, (2007)
J. Just, and H. Siller. Twelfth Congress of the European Society for Research in Mathematics Education (CERME12), volume TWG26 of Congress of the European Society for Research in Mathematics Education (CERME12), Bozen-Bolzano, Italy, (February 2022)