(* Gabriel Braun November 2012 *)
Require Export GeoCoq.Tarski_dev.Ch12_parallel.
Section Quadrilateral.
Context `{TnEQD:Tarski_neutral_dimensionless_with_decidable_point_equality}.

Lemma plgf_not_comm :
forall A B C D, A <> B ->
Parallelogram_flat A B C D ->
~ Parallelogram_flat A B D C /\ ~ Parallelogram_flat B A C D.