proposition: Lemma ncoplanar_perm_20 : forall A B C D, ~ Coplanar A B C D -> ~ Coplanar D B A C.
proof: Proof. intros A B C D HNCop HCop; apply HNCop. apply coplanar_perm_15, HCop. Qed.
imports: GeoCoq.Tarski_dev.Ch08_orthogonality