Require Export GeoCoq.Meta_theory.Parallel_postulates.parallel_postulates.
Require Export GeoCoq.Meta_theory.Parallel_postulates.par_trans_NID.
Section T13.
Context `{TE:Tarski_euclidean}.

Lemma l12_21 : forall A B C D,
TS A C B D ->
(CongA B A C D C A <-> Par A B C D).