G. Davis, and M. Mcgowen. proceedings of the Annual Meeting of the 26th Annual Meeting of the International Group for the Psychology of Mathematics Education (PME), 2, page 273-280. Norwich, UK, University of Norwich, (July 2002)
K. Schewe, and J. Turull Torres. Proceedings of the 2006 Conference on Information Modelling and Knowledge Bases XVII, page 237--244. Amsterdam, The Netherlands, The Netherlands, IOS Press, (2006)