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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 l_6_269 : forall A B C D F E G,
on_perp_d D B A 1 ->
on_foot F D B C ->
on_perp_d E C A (0-1) ->
on_foot G E B C ->
S A B C = S B D F + S C G E.