public function isUsernameAllowed($username, $attribute = 'access')
    {
        $usernameStr = (string) $username;
        $attrStr     = (string) $attribute;
        return (isset($this->allowedUsernames[$usernameStr]) && isset($this->allowedUsernames[$usernameStr][$attrStr]))
                ? $this->allowedUsernames[$usernameStr][$attrStr] : false;
    }