proposition: Lemma sum_O_O : Sum O E E' O O O.
proof: Proof. unfold Sum. assert(O <> E' /\ E <> E'). split; intro ; subst E'; apply grid_ok; Col. split. spliter. unfold Ar2. repeat split; Col. exists O. exists O. repeat split;try (apply pj_trivial). Col. Qed.
imports: GeoCoq.Tarski_dev.Ch13_6_Desargues_Hessenberg