induction l; intros.
inversion_clear H0...
inversion_clear H.
inversion_clear H0...
apply NoDup_cons...
intro.
apply H1.
apply SkipList_incl with l0...
Qed.

Lemma SkipList_trans (y x: list T): SkipList x y -> forall z, SkipList y z -> SkipList x z.
Proof with auto.