code: def testAckFailed(self):
      comm = self.COMM
      size = comm.Get_size()
      num_acked = comm.Ack_failed(0)
      self.assertEqual(num_acked, 0)
      num_acked = comm.Ack_failed(size)
      self.assertEqual(num_acked, 0)
      num_acked = comm.Ack_failed()
      self.assertEqual(num_acked, 0)
  
imports: from mpi4py import MPI
  import mpiunittest
  import warnings
  import struct
label_desc: Use image node_id 6 for calling the BaseTestULFM obj's underlying member method code with example usage: obj.testAckFailed() without return types