proposition: Lemma ang_exists : forall A B C, A <> B -> C <> B -> exists a, Q_CongA a /\ a A B C.
proof: Proof. intros. exists (fun D E F => CongA A B C D E F). split. unfold Q_CongA. exists A. exists B. exists C. split; auto. split; auto. intros. split. auto. auto. apply conga_refl; auto. Qed.
imports: GeoCoq.Tarski_dev.Ch13_2_length