code: def delete(self, subnet_id):
          
          # 1 : show subnet
          subnet = self.client.describe_subnets(
              SubnetIds=[subnet_id]).get('Subnets')[0]
          vpc_id = subnet.get('VpcId')
          # 2 : delete subnet
          self.client.delete_subnet(SubnetId=subnet_id)
          # 3 : delete vpc
          return self.client.delete_vpc(VpcId=vpc_id)
docs: This is bad delete function
          because one vpc can have more than one subnet.
          It is Ok if user only use CAL for manage cloud resource
          We will update ASAP.