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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 th6_239 : forall A B C D P Q R A1 D1 J,
is_midpoint Q B C ->
is_midpoint P B A ->
is_midpoint R C D ->
inter_ll A1 D Q B R ->
inter_ll D1 A Q C P ->
inter_ll J A A1 D D1 ->
parallel A J A1 J ->
A<>J ->
A1<>J ->
A**J/A1**J=- (2+1).