(** * Proposition 10 To bisect a given finite straight line. *)
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(** * Proposition 13 If a straight line set up on a straight line make angles, it will make either two right angles or angles equal to two right angles. *)
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Lemma prop_13 : forall A B C D P Q R, A <> B -> B <> C -> B <> D -> Bet C B D ->
P <> Q -> Q <> R -> Per P Q R ->
SumA C B A A B D C B D /\ SumA P Q R P Q R C B D.