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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_224 : forall A B C D N M P r,
is_midpoint N A C ->
is_midpoint M B D ->
on_line_d P N M r ->
S P A B + S P C D = S P D A + S P B C.