proposition: Lemma anga_distinct: forall a A B C, Q_CongA_Acute a -> a A B C -> A <> B /\ C <> B.
proof: Proof. intros. assert(Ang_Acute A B C a). split; auto. apply (is_anga_distinct _ _ _ a); auto. Qed.
imports: GeoCoq.Tarski_dev.Ch13_2_length Setoid