Proof with simpl; auto.
revert l2; induction l1... intros l2; inversion 1. intros l2 H1.
induction l2... remember (cmp a a0) as b0; destruct b0...
apply (Hcmp a a0) in Heqb0; subst.
inversion H1; try solve[left; auto]. right...
right; apply IHl2... inversion H1; try solve[left; auto]. right; auto.
Qed.

(* Specifications: *)
Lemma listd_merge_un2 l1 l2 s :
listd f un FF l2 s -> listd f un FF (merge cmp l1 l2) s.