(** 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 l12_18_a :
forall A B C D P,
Cong A B C D -> Cong B C D A -> ~Col A B C ->
B <> D -> Col A P C -> Col B P D ->
Par A B C D.