proposition: Lemma col2_orth_at__orth_at : forall A B C P Q U V X, U <> V -> Col P Q U -> Col P Q V -> Orth_at X A B C P Q -> Orth_at X A B C U V.
proof: Proof. intros A B C P Q U V X HUV HU HV [HNCol [HPQ [HX1 [HX2 HX3]]]]. repeat split; trivial. apply (col3 P Q); auto. intros D W HD HW. apply HX3; [|apply (colx U V)]; assumption. Qed.
imports: GeoCoq.Tarski_dev.Ch10_line_reflexivity_2