R. Masoon, and J. Macfeetors. proceedings of 2002 annual meeting of the Canadian Mathematics Education Study Group, page 143--144. Queen's University, (May 2002)
R. Goldstein, and D. Pratt. Proceedings of the 25th Conference of the International Group for the Psychology of Mathematics Education (PME25), 3, page 49--56. PO Box 9432, 3506 GK Utrecht, The Netherlands, Freudenthal Institute, Faculty of Mathematics and Computer Science, Uthrect University, (July 2001)