(** * [add] *)

Fixpoint add (k : key) (x : elt) (s : t elt) {struct s} : t elt :=
  match s with
   | nil => (k,x) :: nil
   | (k',y) :: l => if X.eq_dec k k' then (k,x)::l else (k',y)::add k x l
  end.

Lemma add_3 : forall m x y e e', ~ X.eq x y -> MapsTo y e (add x e' m) -> MapsTo y e m.