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