proposition: Lemma plg_permut : forall A B C D, Parallelogram A B C D -> Parallelogram B C D A.
proof: Proof. intros A B C D HPara. elim HPara; clear HPara; intro HPara. { left; apply plgs_permut; Col. } { right; apply plgf_permut; Col. } Qed.
imports: GeoCoq.Tarski_dev.Ch12_parallel_inter_dec GeoCoq.Tarski_dev.Annexes.Tagged_predicates