proposition: Lemma osp_transitivity : forall A B C P Q R, OSP A B C P Q -> OSP A B C Q R -> OSP A B C P R.
proof: Proof. intros A B C P Q R [S [HPS HQS]] HQR. exists S; split; [|apply l9_41_2 with Q]; assumption. Qed.
imports: GeoCoq.Tarski_dev.Ch08_orthogonality GeoCoq.Tarski_dev.Annexes.coplanar