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(* Formalization of the Chou, Gao and Zhang's decision procedure.          *)
(* Julien Narboux (Julien@narboux.fr)                                      *)
(* LIX/INRIA FUTURS 2004-2006                                              *)
(* University of Strasbourg 2008                                           *)
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Theorem Desargues :
forall A B C X A' B' C' : Point, A' <>C' -> A' <> B' ->
on_line A' X A ->
on_inter_line_parallel B' A' X B A B ->
on_inter_line_parallel C' A' X C A C ->
parallel B C B' C'.