proposition: Lemma orth_at__ncop : forall A B C P X, Orth_at X A B C X P -> ~ Coplanar A B C P.
proof: Proof. intros A B C P X HOrth. assert (Hd := HOrth); apply orth_at_distincts in Hd; spliter. apply orth_at__ncop2 with X X; auto. Qed.
imports: GeoCoq.Tarski_dev.Ch10_line_reflexivity_2