proposition: Lemma col2_os__os : forall A B C D X Y, C <> D -> Col A B C -> Col A B D -> OS A B X Y -> OS C D X Y.
proof: Proof. intros A B C D X Y HCD HColC HColD Hos. destruct Hos as [Z [Hts1 Hts2]]. exists Z. split; apply (col_preserves_two_sides A B); auto. Qed.
imports: GeoCoq.Tarski_dev.Ch08_orthogonality GeoCoq.Tarski_dev.Annexes.coplanar