Proof using.
intros. do_in_app. intuition.
find_eapply_lem_hyp Prefix_In; [|eauto].
match goal with
| H : sorted (?l ++ ?l'), _ : In _ ?l, _ : In _ ?l' |- _ =>
eapply sorted_app_in_in in H
end; eauto. omega.
Qed.

(* Specifications: *)
Lemma sorted_app_in2_in2 :
forall l1 l2 e e',
sorted (l1 ++ l2) ->
In e' (l1 ++ l2) ->
In e l2 ->
eIndex e' = eIndex e ->
In e' l2.