code: def test_sentences(self):
      corpus = PlainTextCorpus()
      corpus.load(self.plaintext_folder)
      self.assertEqual(list, type(corpus.documents[0].sentences))
  
file_path: underthesea/tests/corpus/test_plaintext.py
imports: import os
  import shutil
  from os import listdir
  from unittest import TestCase
  from os.path import join
  import sys
  from underthesea.corpus import PlainTextCorpus
label_desc: Use image node_id 4 for calling the TestPlainTextCorpus obj's underlying member method code with example usage: obj.test_sentences() without return types