Department of Computer Science, University of Bari, Via Orabona 4, 70125 Bari, Italy. turi@di.uniba.it|Department of Computer Science, University of Bari, Via Orabona 4, 70125 Bari, Italy. turi@di.uniba.it|Department of Computer Science, University of Bari, Via Orabona 4, 70125 Bari, Italy. turi@di.uniba.it|Department of Computer Science, University of Bari, Via Orabona 4, 70125 Bari, Italy. turi@di.uniba.it|Department of Computer Science, University of Bari, Via Orabona 4, 70125 Bari, Italy. turi@di.uniba.it|Department of Computer Science, University of Bari, Via Orabona 4, 70125 Bari, Italy. turi@di.uniba.it