proposition: Lemma permut_get_3 : forall l1 l2 l3 l4, (l1 \u l2 \u l3 \u l4) = (l2 \u l3 \u l1 \u l4).
proof: Proof. Proof using. intros. do 2 rewrite (union_assoc l2). apply permut_get_2. Qed.
imports: TLC(LibTactics LibLogic LibReflect LibRelation LibList LibInt LibNat LibOperation LibEpsilon LibSet LibMonoid)