intros; intro.
unfold Seqtl in |- *.
simpl in |- *.
case x.
intros.
apply Ap_comp; auto with algebra.
Qed.
Hint Resolve modify_tl_tl: algebra.

Lemma Seqtl_modify_seq :
forall (A : Setoid) (n : Nat) (v : seq (S n) A) (a : A) (H : 0 < S n),
modify_seq v (Build_finiteT H) a =' a;; Seqtl v in _.