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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_69 : forall A B C D E F,
on_foot F C A B ->
on_foot E B A C ->
on_foot D A B C ->
eq_angle E D C B D F.