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

(* Specifications: *)
Theorem app_length : forall l1 l2 : natlist,
length (l1 ++ l2) = (length l1) + (length l2).