proposition: Lemma anga_sym : forall a A B C, Q_CongA_Acute a -> a A B C -> a C B A.
proof: Proof. intros. unfold Q_CongA_Acute in H. ex_and H A0. ex_and H1 B0. ex_and H C0. assert(HH:= H1 A B C). destruct HH. apply H3 in H0. apply conga_right_comm in H0. assert(HH:= H1 C B A). destruct HH. apply H4. auto. Qed.
imports: GeoCoq.Tarski_dev.Ch13_2_length Setoid