private static function extractFieldContent()
    {
        if (in_array(self::$line, self::FIELD_BREAKS, true)) {
            self::$line = false;

            return null;
        }

        list($content, self::$line) = explode(self::$delimiter, self::$line, 2) + [1 => false];
        if (false === self::$line) {
            return rtrim($content, "\r\n");
        }

        return $content;
    }