intros.
do 3 intro.
destruct (le_lt_or_eq b i H0).
intros.
unfold IndexSeq in H.
right.
apply H...
simpl in H1.
omega.
subst.
left...
Qed.

(* Specifications: *)
Lemma indices: exists tl, ol = map subscript tl /\ IndexSeq 0 tl.
Proof with auto.