auto with algebra.
Qed.
Hint Resolve head_unfolding1 head_unfolding2: algebra.
Hint Extern 0 (head _ =' _ in _) => unfold head in |- *: algebra.
Hint Extern 0 (_ =' head _ in _) => unfold head in |- *: algebra.

Lemma head_cons_inv : forall v : seq n A, head (a;; v) =' a in _.
auto with algebra.