public function addAllowedUsername($username, $attribute = 'access')
    {
        $usernameStr = (string) $username;
        if (!isset($this->allowedUsernames[$usernameStr])) {
            $this->allowedUsernames[$usernameStr] = array();
        }
        $this->allowedUsernames[$usernameStr][(string) $attribute] = true;
        return $this;
    }