Proof with auto.
intros H1; split.
apply listd_In_pred. intros a H2.
apply (listd_In_inv_pred _ a) in H1...
rewrite <-merge_elems. left... auto.
apply listd_In_pred. intros a H2.
apply (listd_In_inv_pred _ a) in H1...
rewrite <-merge_elems. right... auto.
Qed.

(* Specifications: *)
Lemma listd_merge_un1 l1 l2 s :
listd f un FF l1 s -> listd f un FF (merge cmp l1 l2) s.