(* QuickChick app_assoc. *)

Fixpoint rev (l:natlist) : natlist :=
  match l with
  | nil    => nil
  | h :: t => rev t ++ [h]
  end.

Theorem rev_app_distr: forall l1 l2 : natlist, rev (l1 ++ l2) = rev l2 ++ rev l1.