M. Cerulli, A. Chioccariello, and E. Lemut. 5th CERME conference - congress of European Society for Research in Mathematics Education, Larnaca, Cyprus, (2007)
A. Cypher, and D. Smith. CHI '95: Proceedings of the SIGCHI conference on Human factors in computing systems, page 27-34. New York, NY, ACM Press/Addison-Wesley Publishing Co., (1995)
D. Ingalls, T. Kaehler, J. Maloney, S. Wallace, and A. Kay. OOPSLA '97: Proceedings of the 12th ACM SIGPLAN conference on Object-oriented programming, systems, languages, and applications, page 318-326. New York, NY, ACM Press, (1997)