1. Department of Computer Science, University of Oviedo, Gijón, Spain. E-mails: E-mail: palaciosjuan@uniovi.es, E-mail: crvela@uniovi.es, E-mail: puente@uniovi.es
2. Department of Mathematics, Statistics and Computing, University of Cantabria, Santander, Spain. E-mail: E-mail: ines.gonzalez@unican.es