public function ceiling($key)
    {
        if ($this->root) {
            $ceiling = $this->root->find($key, $this->comparator, 1);
        } else {
            $ceiling = null;
        }

        if ($ceiling) {
            return $ceiling;
        } else {
            throw new \OutOfBoundsException('Ceiling element unexisting');
        }
    }