intros.
assert (HPar := plg_par A B C D H H0 H1); spliter...
Qed.

(* Specifications: *)
Lemma plg_par_2 : forall A B C D, A <> B -> B <> C -> Parallelogram A B C D -> Par A D B C.
Proof with finish.
Lemma plgs_pars_1: forall A B C D : Tpoint, Parallelogram_strict A B C D -> Par_strict A B C D.
Proof with finish.