Section Saccheri.
Context `{TnEQD:Tarski_neutral_dimensionless_with_decidable_point_equality}.
(** The two following lemmas come from Theorem 21.10 *)
(** The five following lemmas come from Theorem 21.8 *)

Lemma lta123234_os_per2__lt : forall A B C D,
Per B A D -> Per A D C ->
OS A D B C -> LtA A B C B C D ->
Lt D C B A.