Proof using.
intros. find_eapply_lem_hyp maxIndex_le; intuition; eauto.
break_exists. intuition.
unfold contiguous_range_exact_lo in *.
intuition.
find_insterU. conclude_using eauto. omega.
Qed.

Lemma sorted_app_in_in :
forall l1 l2 e e',
sorted (l1 ++ l2) ->
In e l1 ->
In e' l2 ->
eIndex e' < eIndex e.