[1] Univ Pisa, Dept Comp Sci, Pisa, Italy
[2] Fraunhofer Inst IAIS, St Augustin, Germany
[3] City Univ London, London, England
[4] CNR, IIT, Pisa, Italy
[5] CNR, ISTI, Pisa, Italy
[6] Univ Clermont Auvergne, CNRS, IRD, CERDI, Clermont Ferrand, France
[7] Scuola Normale Super Pisa, Pisa, Italy
[8] Univ Tartu, Tartu, Estonia