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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 Menelaus :
forall A B C X Y D E F : Point,
inter_ll D B C X Y ->
inter_ll E A C X Y ->
inter_ll F A B X Y ->
F <> B ->
D <> C ->
E <> A ->
parallel A F F B ->
parallel B D D C ->
parallel C E E A ->
(A ** F / F ** B) * (B ** D / D ** C) * (C ** E / E ** A) = - (1).