Corollary Zlength_upto : forall n, Zlength (upto n) = Z.of_nat n.

Lemma Znth_combine : forall {A B} {a: Inhabitant A} {b: Inhabitant B} i (l1: list A) (l2: list B), Zlength l1 = Zlength l2 -> Znth i (combine l1 l2) = (Znth i l1, Znth i l2).