5 pages, 1 figure.-- Contributed to: ICALT'08 - Eighth IEEE International Conference on Advanced Learning Technologies (Santander, Cantabria, Spain, Jul 1-5, 2008).
Later published as paper (in Spanish) in: Revista Iberoamericana de Tecnologías del Aprendizaje (IEEE-RITA), 2009, v. 4, n. 2, p. 147-154, http://e-archivo.uc3m.es/handle/10016/8962