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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_218 : forall A B C D E F G H,
is_midpoint E A B  ->
is_midpoint F B C ->
is_midpoint G C D ->
is_midpoint H D A ->
2* S4 E F G H = S4 A B C D.