(*****   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 rev_sublist: forall {A} lo hi (al: list A), 0 <= lo <= hi -> hi <= Zlength al -> rev (sublist lo hi al) = sublist (Zlength al - hi) (Zlength al - lo) (rev al).