Proof using.
unfold uniqueIndices.
induction ys; intros.
- simpl in *. break_match; intuition.
- simpl in *. break_match; intuition.
+ simpl. constructor.
+ subst. simpl in *. invc H0. constructor; auto.
intro. apply H3.
eapply subseq_In; eauto.
apply subseq_map. auto.
+ subst. invc H0. eauto.
Qed.

(* Specifications: *)
Lemma subseq_findGtIndex :
forall xs i,
subseq (findGtIndex xs i) xs.