Proof using.
intros.
eapply removeAfterIndex_same_sufficient; eauto.
intros.
unfold contiguous_range_exact_lo in *. intuition.
specialize (H7 (eIndex e)).
intuition. find_copy_apply_hyp_hyp.
repeat conclude_using omega.
break_exists. intuition.
symmetry in H9. copy_apply H4 H10; try omega.
eapply rachet with (xs := l'); eauto using sorted_uniqueIndices.
Qed.

Lemma thing2 :
forall l l' i,
l <> [] ->
Prefix l l' ->
sorted l' ->
contiguous_range_exact_lo l i ->
contiguous_range_exact_lo l' 0 ->
l ++ (removeAfterIndex l' i) = l'.