code: def extract_char_tokens(word_counts):
      """Extracts all single-character tokens from word_counts.
  
      Args:
        word_counts: list of (string, int) tuples
  
      Returns:
        set of single-character strings contained within word_counts
      """
  
      seen_chars = set()
      for word, _ in word_counts:
          for char in word:
              seen_chars.add(char)
      return seen_chars
  
file_path: text/tensorflow_text/tools/wordpiece_vocab/wordpiece_tokenizer_learner_lib.py
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import collections
  from typing import List, Optional
  import numpy
label_desc: Use image node_id 1 for calling a global function with example usage: extract_char_tokens(word_counts) and returns: seen_chars