proposition: Lemma col_cop_orth__orth_at : forall A B C U V X, Orth A B C U V -> Coplanar A B C X -> Col U V X -> Orth_at X A B C U V.
proof: Proof. intros A B C U V X [Y [HNCol [HUV [HY1 [HY2 HY3]]]]] HX1 HX2. repeat split; trivial. replace X with Y; [assumption|]. apply eq_sym, l8_8; auto. Qed.
imports: GeoCoq.Tarski_dev.Ch10_line_reflexivity_2