(* not this version! Lemma sublist_list_repeat {A} i j k (v:A) (I: 0<=i) (IJK: (Z.to_nat i <= Z.to_nat j <= k)%nat): sublist i j (list_repeat k v) = list_repeat (Z.to_nat (j-i)) v. *)

Lemma skipn_In {A} (x:A): forall l n, In x (skipn n l) -> In x l.