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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 rectangle_1 : forall A B C D,
on_perp C B A ->
on_parallel_d D C A B (0-1) ->
perp A D  A B.