def _last_index(self, s):
        self.set_index(max(0, self._find_one_after_last_labeled_index() - 1))
def kth(root, k):
    if k < size(root.left):
        return kth(root.left, k)
    elif k > size(root.left):
        return kth(root.right, k - size(root.left) - 1)
    return root.key