anchor.diff:  
       return(hosts)
   
  +def ec2_classes(stackname):
  +    client = boto3.client('ec2', region_name=region)
  +
  +    response = client.describe_instances(
  +           Filters=[
  +              {'Name': 'tag:aws_stackname', 'Values': [stackname]},
  +              {'Name': 'instance-state-name','Values': ['running']}
  +           ]
  +       )
  +
  +    nodes = response['Reservations']
  +
  +    classes = []
  +    for instance in nodes:
  +    classes.append(filter(lambda t: t['Key'] == 'aws_migration', instance['Instances'][0]['Tags'])[0]['Value'])
  +
  +    return(sorted(set(classes)))
  +
   def get_hostname_by_id(instance_id):
       client = boto3.client('ec2', region_name=region)
       response = client.describe_instances(InstanceIds=[instance_id])
  
positive.code:     dest='single_node',
  )
  
  if __name__ == '__main__':
      main()
  
  
negative.code: def main():
      opts, args = parser.parse_args()
  
      if opts.classes:
          classes = ec2_classes(stackname)
          print("\n".join(classes))
          exit(0)
  
      elif opts.node_class:
          if any('-' in c for c in opts.node_class):
              parser.error('Node classes should not use hyphens. Hint: Try an underscore')
  
  