def add_source_identifier_to_subscription(SubscriptionName=None,
    SourceIdentifier=None):
    """
    Adds a source identifier to an existing RDS event notification subscription.
    See also: AWS API Documentation
    
    Examples
    This example add a source identifier to an event notification subscription.
    Expected Output:
    
    :example: response = client.add_source_identifier_to_subscription(
        SubscriptionName='string',
        SourceIdentifier='string'
    )
    
    
    :type SubscriptionName: string
    :param SubscriptionName: [REQUIRED]
            The name of the RDS event notification subscription you want to add a source identifier to.
            

    :type SourceIdentifier: string
    :param SourceIdentifier: [REQUIRED]
            The identifier of the event source to be added. An identifier must begin with a letter and must contain only ASCII letters, digits, and hyphens; it cannot end with a hyphen or contain two consecutive hyphens.
            Constraints:
            If the source type is a DB instance, then a DBInstanceIdentifier must be supplied.
            If the source type is a DB security group, a DBSecurityGroupName must be supplied.
            If the source type is a DB parameter group, a DBParameterGroupName must be supplied.
            If the source type is a DB snapshot, a DBSnapshotIdentifier must be supplied.
            

    :rtype: dict
    :return: {
        'EventSubscription': {
            'CustomerAwsId': 'string',
            'CustSubscriptionId': 'string',
            'SnsTopicArn': 'string',
            'Status': 'string',
            'SubscriptionCreationTime': 'string',
            'SourceType': 'string',
            'SourceIdsList': [
                'string',
            ],
            'EventCategoriesList': [
                'string',
            ],
            'Enabled': True|False,
            'EventSubscriptionArn': 'string'
        }
    }
    
    
    :returns: 
    (string) --
    
    """
    pass
