Section Book_1_part_3.
Context `{T2D:Tarski_2D}.
Context `{TE:@Tarski_euclidean Tn TnEQD}.
(** * Proposition 29 A straight line falling on parallel straight lines makes the alternate angles equal to one another, the exterior angle equal to the interior and opposite angle, and the interior angles on the same side equal to two right angles. *)
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(** * Proposition 30 Straight lines parallel to the same straight line are also parallel to one another. *)
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Lemma prop_30 : forall A B C D E F, Par A B C D -> Par C D E F ->
Par A B E F.