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

Lemma Permutation_elt : forall l1 l2 l1' l2' (a:A), Permutation (l1 ++ l2) (l1' ++ l2') -> Permutation (l1 ++ a :: l2) (l1' ++ a :: l2').