#[export] Instance Permutation_app' :
Proper (@Permutation A ==> @Permutation A ==> @Permutation A) (@app A).

(* Specifications: *)
Lemma Permutation_add_inside : forall a (l l' tl tl' : list A),
Permutation l l' -> Permutation tl tl' ->
Permutation (l ++ a :: tl) (l' ++ a :: tl').
Lemma Permutation_cons_append : forall (l : list A) x,
Permutation (x :: l) (l ++ x :: nil).