Păun, Gheorghe, Institute of Mathematics of the Romanian Academy PO Box 1-764, 014700 Bucure¸sti, Romania and Research Group on Natural Computing Department of Computer Science and Artificial Intelligence University of Sevilla Avda. Reina Mercedes s/n, 41012 Sevilla, Spain E-mail: george.paun@imar.ro, gpaun@us.es, Romania