def hidden(self):
        """return list of hidden members"""
        members = [self.member_info(item["_id"]) for item in self.members()]
        result = []
        for member in members:
            if member['rsInfo'].get('hidden'):
                server_id = member['server_id']
                result.append({
                    '_id': member['_id'],
                    'host': self._servers.hostname(server_id),
                    'server_id': server_id})
        return result
return list of hidden members