proposition: Lemma l8_14_3 : forall A B C D X Y, Perp_at X A B C D -> Perp_at Y A B C D -> X=Y.
proof: Proof. intros. eapply l8_14_2_1b. apply H. unfold Perp_at in H0. intuition. eapply l8_12 in H0. unfold Perp_at in H0. intuition. Qed.
imports: GeoCoq.Tarski_dev.Ch07_midpoint GeoCoq.Tactics.Coinc.ColR