(** 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 diff_not_col_par_not_col : forall A B P Q, A<>B -> ~ Col A Q P -> parallel A B P Q -> ~ Col A B P.