proposition: Lemma length_pos_or_null : forall O E E' A B AB, Length O E E' A B AB -> Ps O E AB \/ A = B.
proof: Proof. intros. induction(eq_dec_points A B). { right; auto. } { apply length_Ps in H. left; auto. intro. subst AB. unfold Length in H. spliter. treat_equalities. tauto. } Qed.
imports: GeoCoq.Tarski_dev.Ch14_order