proposition: Lemma sum_B_exists : forall O E E' B AB, Ar2 O E E' B AB O -> exists A, Sum O E E' A B AB.
proof: Proof. intros. unfold Ar2 in *. spliter. assert(HH:=diff_exists O E E' AB B H H1 H0). ex_and HH A. exists A. apply diff_sum in H3. apply sum_comm; auto. Qed.
imports: GeoCoq.Tarski_dev.Ch13_6_Desargues_Hessenberg