proposition: Lemma sum_stable : forall O E E' A B C S1 S2 , A = B -> Sum O E E' A C S1 -> Sum O E E' B C S2 -> S1 = S2.
proof: Proof. intros. subst B. apply (sum_uniqueness O E E' A C); auto. Qed.
imports: GeoCoq.Tarski_dev.Ch13_6_Desargues_Hessenberg