(** * [add] *)

Fixpoint add (k : key) (x : elt) (s : t elt) {struct s} : t elt :=
 match s with
  | nil => (k,x) :: nil
  | (k',y) :: l =>
     match X.compare k k' with
      | LT _ => (k,x)::s
      | EQ _ => (k,x)::l
      | GT _ => (k',y) :: add k x l
     end
 end.

Lemma add_Inf : forall (m:t elt)(x x':key)(e e':elt), Inf (x',e') m -> ltk (x',e') (x,e) -> Inf (x',e') (add x e m).