1 LAGA, Université Paris XIII, Villetaneuse 93430, France, halpern@math.univ-paris13.fr 2 LAGA, Université Paris XIII, Villetaneuse 93430, France; CSCAMM, University of Maryland College Park, MD 20742 USA, japhet@cscamm.umd.edu, the first two authors are partially supported by french ANR (COMMA) and GdR MoMaS. 3 Département de mathématiques et applications, Ecole Normale supérieure, 45 rue d’Ul...