let neg_interval r =
match r.F.inf,r.F.sup with
| Some j, None -> {F.inf = None ; F.sup = Some (F.e_ineg j)}
| None , Some j -> {F.inf = Some (F.e_ineg j) ; F.sup = None}
| Some j, Some k ->
{F.inf = Some (F.e_ineg k) ; F.sup = Some (F.e_ineg j)}
| None, None -> r