(*****   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 sublist0_app2 {A : Type} i (al bl : list A): Zlength al <= i <= Zlength al + Zlength bl -> sublist 0 i (al ++ bl) = al ++ sublist 0 (i - Zlength al) bl.