1. Microsoft Research Cambridge, 7, J.J. Thomson Avenue, Cambridge, CB3 0FB, UK
2. Institute of Mathematics of the Romanian Academy, PO Box 1-764, 014700 Bucureşti, Romania
3. Research Group on Natural Computing, Department of Computer Science and Artificial Intelligence, University of Sevilla, Avda. Reina Mercedes s/n, 41012 Sevilla, Spain