Proof using.
intros.
eapply NoDup_map_elim with (f := eIndex); eauto using findAtIndex_in, removeAfterIndex_in.
apply findAtIndex_index in H0.
apply findAtIndex_index in H1.
congruence.
Qed.

(* Specifications: *)
Lemma subseq_uniqueIndices :
forall ys xs,
subseq xs ys ->
uniqueIndices ys ->
uniqueIndices xs.