proposition: Lemma NoDup_app_swap : forall {A} a b, @NoDup A (a ++ b) -> @NoDup A (b ++ a).
proof: Proof. intros. eapply NoDup_Permutation_rewrite; try apply Permutation_app_comm; assumption. Qed.
imports: Coq.Sorting.Permutation Fiat.Common Coq.Lists.List Fiat.Common.List.ListFacts