1. Department of Computer Science, University of Salamanca, IBSAL/BISITE Research Group. Edificio I+D+I, 37007, Salamanca, Spain. E-mails: chamoso@usal.es, srg@usal.es, fer@usal.es
2. Department of Artificial Intelligence, Technical University of Madrid, Campus Montegancedo, Boadill a del Monte, 28660, Madrid, Spain. E-mail: jbajo@fi.upm.es