(** * [find] *)

Lemma find_1 :  forall m (Hm:Sort m) x e, MapsTo x e m -> find x m = Some e.
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.