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

Lemma Permutation_middle2 : forall l1 l2 l3 a b, Permutation (a :: b :: l1 ++ l2 ++ l3) (l1 ++ a :: l2 ++ b :: l3).