cut (forall (z y: list T), SkipList y z -> forall x, SkipList x y -> SkipList x z).
intros.
apply H with y...
induction z.
intros.
inversion H.
subst.
inversion_clear H0...
intros.
inversion H.
subst.
inversion_clear H0...
apply SkipList_head.
apply IHz with l...
apply SkipList_tail.
apply IHz with l...
subst.
apply SkipList_tail.
apply IHz with y0...
subst.
inversion_clear H0...
Qed.
End contents.
Hint Constructors SkipList.
Hint Resolve SkipList_refl.

(* Specifications: *)
Lemma SkipList_map (A: Set) (x y: list A): SkipList x y -> forall (B: Set) (f: A -> B), SkipList (map f x) (map f y).