Proof using.
intros; induction l; simpl; auto.
- unfold uniqueIndices. simpl. constructor.
- unfold uniqueIndices in *. simpl in *. intuition. constructor; eauto.
intuition. do_in_map. find_apply_hyp_hyp. omega.
Qed.

Lemma findAtIndex_intro' :
forall l i e,
sorted l ->
In e l ->
eIndex e = i ->
findAtIndex l i = Some e.