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