Proof with Geometry.
unfold inter_ll in |- *.
intros.
intuition.
Qed.
Hint Resolve inter_ll_comm1 inter_ll_comm2 inter_ll_comm3 inter_ll_comm4: Geom.

Theorem co_side_elim_2 : forall A B P Q M : Point,
P <> M -> inter_ll M A B P Q -> Q ** M / P ** M = S Q A B / S P A B.