(** We choose the most general notion such that l2_11 is still valid *)
Definition  weak_3_parallelogram (A B C D : Point) : Prop :=
exists O, mid_point O A C /\ mid_point O B D.

Lemma weak_para_1 : forall W S V U,
weak_3_parallelogram W S V U -> weak_3_parallelogram U V S W.