1. Department of Technology of Computers and Communications, Universidad de Extremadura, 10003 Cáceres, Spain
2. Department of Computer Science and Information Systems, Bradley University, Peoria, IL 61625, USA
3. School of Computer Engineering, Pontificia Universidad Católica de Valparaíso, Valparaíso 4059, Chile
4. Department of Computer Sciences, Universidad de Alcalá, 28805 Alcalá de Henares, Spain