intros; unfold Str_nth; rewrite Str_nth_tl_plus;
trivial with datatypes.
Qed.
(** Extensional Equality between two streams  *)
CoInductive EqSt (s1 s2: Stream) : Prop :=
eqst :
hd s1 = hd s2 -> EqSt (tl s1) (tl s2) -> EqSt s1 s2.
(** A coinduction principle *)
Ltac coinduction proof :=
cofix proof; intros; constructor;
[ clear proof | try (apply proof; clear proof) ].
(** Extensional equality is an equivalence relation *)

(* Specifications: *)
Theorem EqSt_reflex : forall s:Stream, EqSt s s.
coinduction EqSt_reflex.