Corollary combine_upd_Znth1 : forall {A B}{d: Inhabitant B} (l1 : list A) (l2 : list B) i x,
   0 <= i < Zlength l1 ->
  Zlength l1 = Zlength l2 ->
   combine (upd_Znth i l1 x) l2 = upd_Znth i (combine l1 l2) (x, Znth i l2).Corollary combine_upd_Znth2 : forall {A B}{d: Inhabitant A} (l1 : list A) (l2 : list B) i x, 0 <= i < Zlength l1 ->
  Zlength l1 = Zlength l2 -> combine l1 (upd_Znth i l2 x) = upd_Znth i (combine l1 l2) (Znth i l1, x).

Lemma length_concat_upd : forall {A} {d: Inhabitant A} l i (l' : list A) (Hi : 0 <= i < Zlength l), length (concat (upd_Znth i l l')) = (length (concat l) + length l' - length (Znth i l))%nat.