(***************************************************************************)
(* Formalization of the Chou, Gao and Zhang's decision procedure.          *)
(* Julien Narboux (Julien@narboux.fr)                                      *)
(* LIX/INRIA FUTURS 2004-2006                                              *)
(* University of Strasbourg 2008                                           *)
(***************************************************************************)

Lemma l6_223 : forall A B C D X Y W,
is_midpoint X C A ->
is_midpoint Y B D ->
inter_ll W B C D A ->
S4 B C D A = (2+2)* S X Y W.