proposition: Lemma lcos2_lg_not_null: forall lp l a b, Lcos2 lp l a b -> ~ Q_Cong_Null l /\ ~ Q_Cong_Null lp.
proof: Proof. intros. unfold Lcos2 in H. ex_and H la. apply lcos_lg_not_null in H. apply lcos_lg_not_null in H0. spliter. split; auto. Qed.
imports: GeoCoq.Tarski_dev.Ch13_3_angles Morphisms