Definition weak_2_parallelogram (A B C D : Point) : Prop :=
  (A=C <-> B=D) /\
  exists O, mid_point O A C /\ mid_point O B D.

(* Specifications: *)
Theorem l2_11a_strong_strong : forall A B C D P Q : Point, weak_2_parallelogram A B C D -> S A P Q + S C P Q = S B P Q + S D P Q.