(***************************************************************************)
(* Formalization of the Chou, Gao and Zhang's decision procedure.          *)
(* Julien Narboux (Julien@narboux.fr)                                      *)
(* LIX/INRIA FUTURS 2004-2006                                              *)
(* University of Strasbourg 2008                                           *)
(***************************************************************************)
Require Export basic_geometric_facts.

(* Specifications: *)
Theorem common_point_not_par_aux :
forall A B C D Y : Point,
Col Y A B ->
Col Y C D ->
A <> B ->
C <> D ->
~ Col D A B ->
~ Col Y A C ->
~ parallel A B C D.