code: def test_category_words(self):
      self.assertEqual(
          ptb.words(categories=["humor", "fiction"])[:6],
          ["Thirty-three", "Scotty", "did", "not", "go", "back"],
      )
  
imports: import unittest
  import pytest
  from nltk.corpus import cess_cat, cess_esp, conll2007, floresta, indian, ptb, sinica_treebank, udhr
  from nltk.tree import Tree
label_desc: Use image node_id 6 for calling the TestPTB obj's underlying member method code with example usage: obj.test_category_words() without return types