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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_subnets(self):
 """find usage for Subnets; return dict of SubnetId to AZ"""

        # subnets per VPC
        subnet_to_az = {}
        subnets = defaultdict(int)
        for subnet in self.conn.describe_subnets()['Subnets']:
            subnets[subnet['VpcId']] += 1
            subnet_to_az[subnet['SubnetId']] = subnet['AvailabilityZone']
        for vpc_id in subnets:
            self.limits['Subnets per VPC']._add_current_usage(
                subnets[vpc_id],
                aws_type='AWS::EC2::VPC',
                resource_id=vpc_id
            )
        return subnet_to_az