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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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Lemma l6_17 : forall A B R P Q S C F,
on_line P B R ->
on_line Q A R ->
inter_ll S A P B Q ->
inter_ll C R S A B ->
inter_ll F P Q A B ->
parallel A C B C ->
parallel A F B F ->
B<>C -> B<>F ->
A**C / B**C = - A**F / B**F.