Proof with Tarski.
intros.
unfold le.
exists A.
intuition.
Qed.
Hint Resolve l5_1 l5_2 l5_3 : Between.

(* Specifications: *)
Lemma l6_16_1 : forall P Q S X,
P<>Q -> S<>P -> Col S P Q -> (Col X P Q -> Col X P S).