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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 CreateBlockDeviceMap(self, image_id, instance_type):
 """ If you launch without specifying a manual device block mapping, you may not get all the ephemeral devices available to the given instance type. This will build one that ensures all available ephemeral devices are mapped. """

      # get the block device mapping stored with the image
      image = self.ec2.get_image(image_id)
      block_device_map = image.block_device_mapping
      assert(block_device_map)
      # update it to include the ephemeral devices      
      # max is 4... is it an error for instances with fewer than 4 ?  
      # see: http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/
      # InstanceStorage.html#StorageOnInstanceTypes
      ephemeral_device_names = ['/dev/sdb', '/dev/sdc', '/dev/sdd', '/dev/sde']
      for i, device_name in enumerate(ephemeral_device_names):
        name = 'ephemeral%d' % (i)
        bdt = blockdevicemapping.BlockDeviceType(ephemeral_name = name)
        block_device_map[device_name] = bdt
      return block_device_map