Proof with Geometry.
intros.
cases_col D A B.
unfold parallel, S4 in |- *.
cut (Y = D).
intro.
assert (S A B D = S D A B)...
rewrite H6.
rewrite H4.
unfold not in |- *.
intro.
NormalizeRing H7.
clear H H0.
rewrite H5 in H3.
clear H5.
clear H6.
assert (Col A B D)...
assert (Col A B C)...
assert (Col A D C); eauto with Geom.
assert (Col A B Y)...
assert (Col A B D)...
assert (Col A Y D); eauto with Geom.
assert (A <> Y); eauto with Geom.
cases_equality Y D.
auto.
assert (Col Y D A)...
assert (Col Y D C)...
assert (Col Y A C); eauto with Geom.
intuition.
eapply common_point_not_par_aux; apply H || auto.
Qed.
Definition parallelogram (A B C D : Point) : Prop :=
parallel A B C D /\ parallel B C A D /\ ~ Col A B C.

Theorem l1_24 :
forall A B C D O : Point,
parallelogram A B C D ->
Col A C O ->
Col B D O ->
A ** O = O ** C.