[1] Univ Bordeaux, Gretha, UMR 5113, IMB,UMR 5251, F-33400 Talence, France
[2] CNRS, Gretha, UMR 5113, IMB,UMR 5251, F-33400 Talence, France
[3] INRIA Bordeaux Sud Ouest, Team CQFD, F-33400 Talence, France
[4] Univ Bordeaux, IMB, UMR 5251, F-33400 Talence, France
[5] CNRS, IMB, UMR 5251, F-33400 Talence, France
[6] INRIA Bordeaux Sud Ouest, Team CQFD, F-33400 Talence, France
[7] Univ Lille 1, Lab Paul Painleve, UMR 8524, F-59655 Villeneuve Dascq, France
[8] CNRS, Lab Paul Painleve, UMR 8524, F-59655 Villeneuve Dascq, France