proposition: Lemma thales_theorem : forall A B C M, Midpoint M A B -> Cong M A M C -> Per A C B.
proof: Proof. apply rah__thales_postulate. unfold postulate_of_right_saccheri_quadrilaterals; apply right_saccheris. Qed.
imports: GeoCoq.Tarski_dev.Annexes.circles GeoCoq.Tarski_dev.Annexes.half_angles GeoCoq.Tarski_dev.Ch12_parallel_inter_dec