Definition cons_inj {A} {a1 a2} {n} {v1 v2 : t A n}
 (eq : a1 :: v1 = a2 :: v2) : a1 = a2 /\ v1 = v2 :=
   match eq in _ = x return caseS _ (fun a2' _ v2' => fun v1' => a1 = a2' /\ v1' = v2') x v1
   with | eq_refl => conj eq_refl eq_refl
   end.

Lemma fold_left_right_assoc_eq {A B} {f: A -> B -> A} (assoc: forall a b c, f (f a b) c = f (f a c) b) {n} (v: t B n): forall a, fold_left f a v = fold_right (fun x y => f y x) v a.