Proof using.
intros.
assert (x = x').
- eapply uniqueIndices_elim_eq; eauto.
- subst. auto.
Qed.

(* Specifications: *)
Lemma findAtIndex_intro :
forall l i e,
sorted l ->
In e l ->
eIndex e = i ->
uniqueIndices l ->
findAtIndex l i = Some e.