auto with algebra.
Qed.
(** %\label{head}% *)
Definition head (A : Setoid) (n : Nat) (v : seq (S n) A) :=
v (Build_finiteT (lt_O_Sn n)).

Lemma head_comp :
forall (A : Setoid) (n : Nat) (v v' : seq (S n) A),
v =' v' in _ -> head v =' head v' in _.