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 lt4321_os_per2__lta : forall A B C D,
Per B A D -> Per A D C ->
OS A D B C -> Lt D C B A ->
LtA A B C B C D.