code: def test_punkt_pair_iter(self):
      test_cases = [
          ("12", [("1", "2"), ("2", None)]),
          ("123", [("1", "2"), ("2", "3"), ("3", None)]),
          ("1234", [("1", "2"), ("2", "3"), ("3", "4"), ("4", None)]),
      ]
  
      for test_input, expected_output in test_cases:
          actual_output = [x for x in punkt._pair_iter(test_input)]
  
          assert actual_output == expected_output
  
imports: from typing import List, Tuple
  import pytest
  from nltk.tokenize import LegalitySyllableTokenizer, StanfordSegmenter, SyllableTokenizer, TreebankWordTokenizer, TweetTokenizer, punkt, sent_tokenize, word_tokenize
  from nltk.tokenize.simple import CharTokenizer
label_desc: Use image node_id 15 for calling the TestTokenize obj's underlying member method code with example usage: obj.test_punkt_pair_iter() without return types