intros.
simpl in |- *.
auto with algebra.
Qed.
Hint Resolve modify_hd_hd: algebra.

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