proposition: Lemma sum3_comm_2_3 : forall O E E' A B C S, sum3 O E E' A B C S -> sum3 O E E' A C B S.
proof: Proof. intros. apply sum3_permut in H. apply sum3_comm_1_2 in H. assumption. Qed.
imports: GeoCoq.Tarski_dev.Ch13_6_Desargues_Hessenberg