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 conga_per2_os__cong : forall A B C D,
Per B A D -> Per A D C ->
OS A D B C -> CongA B C D A B C ->
Cong A B C D.