1. King's College London, Department of Mathematics, Strand, London, WC2R 2LS, UK
2. Didaktik der Mathematik, Universitaet Kassel, D-34109, Kassel, Germany
3. Laboratory of Neurobiophysics, University Joseph Fourier—GRENOBLE 1, INSERM U, 318—CHUG Michallon, Grenoble, France