public function verifySettings()
    {
        if ($this->min < 0 && $this->max < 0) {
            $this->setError(
                "You must set at least one of the min or the " .
                "max levels."
            );
        }
        if ($this->max < $this->min && $this->max > -1) {
            $this->setError("The maximum depth is lower than the minimum.");
        }
    }