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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 get_all_dbinstances(self, instance_id=None, max_records=None, marker=None):
 """ Retrieve all the DBInstances in your account. :type instance_id: str :param instance_id: DB Instance identifier. If supplied, only information this instance will be returned. Otherwise, info about all DB Instances will be returned. :type max_records: int :param max_records: The maximum number of records to be returned. If more results are available, a MoreToken will be returned in the response that can be used to retrieve additional records. Default is 100. :type marker: str :param marker: The marker provided by a previous request. :rtype: list :return: A list of :class:`boto.rds.dbinstance.DBInstance` """

        params = {}
        if instance_id:
            params['DBInstanceIdentifier'] = instance_id
        if max_records:
            params['MaxRecords'] = max_records
        if marker:
            params['Marker'] = marker
        return self.get_list('DescribeDBInstances', params,
                             [('DBInstance', DBInstance)])