Department of Computer Science, University of Bologna, Bologna, Italy. dilena@cs.unibo.it|Department of Computer Science, University of Bologna, Bologna, Italy. dilena@cs.unibo.it|Department of Computer Science, University of Bologna, Bologna, Italy. dilena@cs.unibo.it|Department of Computer Science, University of Bologna, Bologna, Italy. dilena@cs.unibo.it|Department of Computer Science, University of Bologna, Bologna, Italy. dilena@cs.unibo.it