(***************************************************************************)
(* 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_15 : forall A B O C D r,
is_midpoint O A B ->
on_line_d C O A r ->
on_line_d D O A (1/r) ->
r <> 0 ->
parallel A C C B ->
parallel D A D B ->
C<>B ->
D<>B ->
A<>D ->
A<>C ->
r+1 <> 0 ->
harmonic A B C D.