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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def add_regional_client(self, region_name):
 """Adds a regional client for the specified region if it does not already exist. :param str region_name: AWS Region ID (ex: us-east-1) """

        if region_name not in self._regional_clients:
            session = boto3.session.Session(region_name=region_name, botocore_session=self.config.botocore_session)
            client = session.client("kms", config=self._user_agent_adding_config)
            self._register_client(client, region_name)
            self._regional_clients[region_name] = client