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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 exercice2_38_3 : forall Y P Q U V A B,
inter_ll Y P Q U V ->
on_line A U V ->
S4 U P V Q <>0 ->
~Col U V Q ->
S A B Y = (S U B V) * (S A P Q) / S4 U P V Q.