simple destruct o; simpl in |- *; trivial.
Qed.
(** A hedge is a set of pairs of message. *)
Definition hedge := Msg -> Msg -> Prop.

(* Specifications: *)
(** In classical logic, it is possible to say if (M,N) is in h or not. *)
Lemma hedge_mem_dec : forall (h : hedge) (M N : Msg), h M N \/ ~ h M N.
intros.