(*****   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 upd_Znth_char {A} n l1 (v:A) l2 w: Zlength l1=n -> upd_Znth n (l1 ++ v :: l2) w = l1 ++ w :: l2.