proposition: Lemma perp2_par : forall A B C D O, Perp2 A B C D O -> Par A B C D.
proof: Proof. intros. unfold Perp2 in H. ex_and H X. ex_and H0 Y. apply (l12_9_2D _ _ _ _ X Y). Perp. Perp. Qed.
imports: GeoCoq.Tarski_dev.Ch12_parallel