Inductive queue : Set :=
  | empty : queue
  | input : data -> queue -> queue.Inductive queue : Set :=
  | empty : queue
  | input : data -> queue -> queue.Inductive queue : Set :=
  | empty : queue
  | input : data -> queue -> queue.Inductive exc : Set :=
  | value : data -> exc
  | error : exc.Inductive exc : Set :=
  | value : data -> exc
  | error : exc.Inductive exc : Set :=
  | value : data -> exc
  | error : exc.Definition last (q : queue) :=
  match q with
  | empty => error
  | input d _ => value d
  end.Fixpoint first (q : queue) : exc :=
  match q with
  | empty => error
  | input d q' =>
      match q' with
      | empty => value d
      | input _ _ => first q'
      end
  end.Fixpoint first_out (q : queue) : queue :=
  match q with
  | empty => empty
  | input d q' =>
      match q' with
      | empty => empty
      | input _ _ => input d (first_out q')
      end
  end.Fixpoint In_queue (d : data) (q : queue) {struct q} : Prop :=
  match q with
  | empty => False
  | input d' q' => d = d' \/ In_queue d q'
  end.

(* Specifications: *)
Lemma not_in_empty : forall d : data, ~ In_queue d empty.