1. INRIA, CNRS, IRISA, University of Rennes , Rennes F-35000, France
2. LIPME, INRAE, CNRS, Université de Toulouse , Castanet-Tolosan F-31326, France
3. Institute of Mathematics, Freie Universität Berlin , Berlin D-14195, Germany
4. Univ. Bordeaux, Bordeaux INP, CNRS, LaBRI, UMR5800 , F-33400 Talence, France