(** We define three operators for building messages. *)
Inductive op : Set :=
  | Pub : op
  | Priv : op
  | Hash : op.(** We define three operators for building messages. *)
Inductive op : Set :=
  | Pub : op
  | Priv : op
  | Hash : op.(** We define three operators for building messages. *)
Inductive op : Set :=
  | Pub : op
  | Priv : op
  | Hash : op.(** We define three operators for building messages. *)
Inductive op : Set :=
  | Pub : op
  | Priv : op
  | Hash : op.

Lemma op_eq_dec : forall o1 o2 : op, {o1 = o2} + {o1 <> o2}.