docstr: Test the utility that returns the union of features in a composite
  distance.
docstr_tok: ["Test","the","utility","that","returns","the","union","of","features","in","a","composite","distance","."]
code: def test_composite_features_extract(self):
      """
      Test the utility that returns the union of features in a composite
      distance.
      """
      dist = copy.deepcopy(self.dist)
      dist.append([["a", "b", "a"], "cosine", 13])
      ans = ["a", "b", "c", "d", "e", "f"]
  
      self.assertItemsEqual(
          ans, tc.distances._util._get_composite_distance_features(dist)
      )
  
imports: from __future__ import print_function
  from __future__ import division
  from __future__ import absolute_import
  import copy
  import unittest
  import numpy
  import turicreate
  from turicreate.toolkits._main import ToolkitError
  from collections import Counter
  import sys
label_desc: Use image node_id 6 for calling the DistanceUtilsTest obj's underlying member method code with example usage: obj.test_composite_features_extract() without return types