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

Theorem l2_11a : forall A B C D P Q : Point, parallelogram A B C D -> S A P Q + S C P Q = S B P Q + S D P Q.