public function setFailureFlag($failureFlag)
    {
        if (!in_array($failureFlag, array(self::FAIL_GRACEFULLY, self::FAIL_WITH_ERROR))) {
            throw new \InvalidArgumentException(
                'The failure flag of an SplClassLoader instance must be one of class "FAIL_" constants!'
            );
        }
        $this->_failureFlag = $failureFlag;
        return $this;
    }