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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 midpoint_elim : forall O A B,
on_line_d O A B (1 / 2) ->
O<>A ->
B ** O / O ** A =1.