1. Institute of Information Science and Technologies “Alessandro Faedo” (ISTI), National Research Council (CNR), Pisa, Italy
2. Institute of Information Science and Technologies “Alessandro Faedo” (ISTI), National Research Council (CNR), Italy and Department of Information Engineering, University of Pisa, Pisa, Italy
3. Department of Philology, Literature and Linguistics - University of Pisa, Pisa, Italy