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

Lemma plgs_one_side :
forall A B C D,
Parallelogram_strict A B C D ->
OS A B C D /\ OS C D A B.