[1] Vienna Univ Technol, Dept Comp Sci, A-1060 Vienna, Austria
[2] Univ Sheffield, Dept Comp Sci, Sheffield S1 4DP, S Yorkshire, England
[3] Acad Romana, Bucharest, Romania
[4] Univ Bucharest, Fac Math & Comp Sci, Bucharest 010014, Romania
[5] Univ Seville, Dept Comp Sci & Artificial Intelligence, E-41012 Seville, Spain