(*****   SUBLIST  ******)

Definition sublist {A} (lo hi: Z) (al: list A) : list A :=
  firstn (Z.to_nat (hi-lo)) (skipn (Z.to_nat lo) al).Definition upd_Znth {A} (i: Z) (al: list A) (x: A): list A :=
   sublist 0 i al ++ x :: sublist (i+1) (Zlength al) al.

Lemma list_repeat_app': forall {A: Type} a b (x:A), 0 <= a -> 0 <= b -> list_repeat (Z.to_nat a) x ++ list_repeat (Z.to_nat b) x = list_repeat (Z.to_nat (a+b)) x.