proposition: Lemma plg_cong_1 : forall A B C D, Parallelogram A B C D -> Cong A B C D.
proof: Proof. intros. apply plg_cong in H. spliter. assumption. Qed.
imports: GeoCoq.Tarski_dev.Ch12_parallel_inter_dec GeoCoq.Tarski_dev.Annexes.Tagged_predicates