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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 _find_usage_subnet_groups(self):
 """find usage for subnet groups"""

        paginator = self.conn.get_paginator('describe_db_subnet_groups')
        for page in paginator.paginate():
            for group in page['DBSubnetGroups']:
                self.limits['Subnets per Subnet Group']._add_current_usage(
                    len(group['Subnets']),
                    aws_type='AWS::RDS::DBSubnetGroup',
                    resource_id=group["DBSubnetGroupName"],
                )