(** We show here that from the axioms of neutral geometry we can deduce the existence of a parallel line. This is important because it shows that axioms of neutral geometry are inconsistent with those of elliptic geometry as elliptic geometry assumes that no parallel lines exist. *)
(** This corresponds to l12_10 in Tarski's book. *)

Lemma conga_cop_inangle_per2__inangle : forall A B C P T,
Per A B C -> InAngle T A B C -> CongA P B A P B C -> Per B P T -> Coplanar A B C P ->
InAngle P A B C.