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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_222 : forall A B C D P Q R S,
is_midpoint P A B ->
is_midpoint Q B C ->
is_midpoint S D A ->
is_midpoint R C D ->
Py A C A + Py B D B = 2* (Py Q S Q + Py P R P).