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

Lemma sorted_app_sorted_app_in1_in2_prefix :
forall l1 l2 l3 l4 e e',
sorted (l1 ++ l3) ->
sorted (l2 ++ l4) ->
Prefix l4 l3 ->
In e l1 ->
In e' (l2 ++ l4) ->
eIndex e' = eIndex e ->
In e' l2.