1. Department of Computer Science, Carlos III University of Madrid, Avda. de la Universidad, 30, 28911, Leganés, Spain. E-mail: {david.griol,josemanuel.molina}@uc3m.es
2. Department of Languages and Computer Systems, University of Granada, CITIC-UGR, C/Pdta, Daniel Saucedo Aranda s/n, 18071, Granada, Spain. E-mail: zoraida@ugr.es