proposition: Theorem permutation_all_permutations : forall l1 l2 : list A, permutation l1 l2 -> In l1 (all_permutations l2).
proof: Proof. intros l1 l2 H; unfold all_permutations in |- *; apply permutation_all_permutations_aux; auto. Qed.
imports: List ListAux