Global Instance Permutation_in' :
Proper (Logic.eq ==> @Permutation A ==> iff) (@In A).

(* Specifications: *)
Lemma Permutation_app_tail : forall (l l' tl : list A),
Permutation l l' -> Permutation (l++tl) (l'++tl).
Lemma Permutation_app_head : forall (l tl tl' : list A),
Permutation tl tl' -> Permutation (l++tl) (l++tl').
Theorem Permutation_app : forall (l m l' m' : list A),
Permutation l l' -> Permutation m m' -> Permutation (l++m) (l'++m').