#[export]
Instance Permutation_cons A :
 Proper (Logic.eq ==> @Permutation A ==> @Permutation A) (@cons A).Implicit Type a : A.Implicit Type l : list A.

(* Specifications: *)
Theorem Permutation_in : forall (l l' : list A) (x : A), Permutation l l' -> In x l -> In x l'.