Proof with Geometry.
intros.
assert (~ Col Q A B).
unfold Col;eauto with Geom.
unfold inter_ll in H0.
DecompAndAll.
apply co_side...
Qed.

(* Specifications: *)
Lemma inter_ll_comm1 : forall P A B C D : Point,
inter_ll P A B C D -> inter_ll P A B D C.