unfold head in |- *.
auto with algebra.
Qed.
Hint Resolve head_comp cons_first_element: algebra.

(** - these two lemmas are mainly useful for the algebra Hints database (hence the names) *)
Lemma head_unfolding1 :
forall v : seq (S n) A,
v (Build_finiteT (lt_O_Sn n)) =' a in _ -> head v =' a in _.