[1] Univ Sao Paulo, Inst Math & Stat, Dept Comp Sci, BR-05508090 Sao Paulo, SP, Brazil
[2] CSIC, IIIA, Campus UAB S-N, Bellaterra 08193, Spain
[3] Univ Paul Sabatier, IRIT, 118 Route Narbonne, F-31062 Toulouse 9, France
[4] Southeast Univ, Sch Comp Sci & Engn, Nanjing 211189, Jiangsu, Peoples R China