proposition: Theorem Permutation_app_inv_r l l1 l2 : Permutation (l1 ++ l) (l2 ++ l) -> Permutation l1 l2.
proof: Proof. rewrite 2 (Permutation_app_comm _ l). apply Permutation_app_inv_l. Qed.
imports: List Setoid Compare_dec Morphisms FinFun PeanoNat