Test CV size as well

    fold_datasets = index_splitter.k_fold_split(ds, K)

    for fold in range(K):

      self.assertTrue(fold_datasets[fold][1].X[0] == fold)

      train_data = set(list(fold_datasets[fold][0].X))

      train, cv = fold_datasets[fold][0], fold_datasets[fold][1]

      self.assertTrue(cv.X[0] == fold)

      train_data = set(list(train.X))

      self.assertFalse(fold in train_data)

      self.assertEqual(K - 1, len(train_data))

      self.assertEqual(K - 1, len(train))

      self.assertEqual(1, len(cv))

  def test_singletask_random_k_fold_split(self):

    """