proposition: Lemma is_null_ang_out : forall a A B C, Q_CongA a -> a A B C -> Q_CongA_Null a -> Out B A C.
proof: Proof. intros. unfold Q_CongA_Null in H1. spliter. assert(HH:= (H2 A B C)). apply HH. auto. Qed.
imports: GeoCoq.Tarski_dev.Ch13_2_length Setoid