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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_3 : forall A B C D I,
on_perp C B A ->
on_parallel_d D C A B (0-1) ->
inter_ll I A C B D ->
parallel A I A C ->
A<> C ->
A <> I ->
A ** I / A ** C = 1 / 2.