(** * [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_2 : forall m x y e e',
~ X.eq x y -> MapsTo y e m -> MapsTo y e (add x e' m).