apply is_permutation_reverse_impl_is_permutation.
rewrite rev_involutive.
rewrite rev_involutive.
trivial.
Qed.

(* Specifications: *)
Lemma is_permutation_cons_snoc : forall (A:Set)(x:A)(xs:list A),(is_permutation A (cons x xs) (app xs (cons x nil))).
intros.