docs: def _convert_word(word):
      """Return the plural form of the word if it exists.
  
      Otherwise return the word itself.
      """
      out = None
  
      # Acronyms.
      if word.isupper():
          out = word + "s?"
      # Proper nouns or word with digits.
      elif word.istitle():
          out = word + "('?s)?"
      elif _contains_digit.search(word):
          out = word
  
      if out is not None:
          return out
  
      # Words with non or anti prefixes.
      if _starts_with_non.search(word):
          word = "non-?" + _capitalize_first_letter(_convert_word(word[3:]))
      elif _starts_with_anti.search(word):
          word = "anti-?" + _capitalize_first_letter(_convert_word(word[4:]))
  
      if out is not None:
          return _capitalize_first_letter(out)
  
      # A few invariable words.
      if word in current_app.config["CLASSIFIER_INVARIABLE_WORDS"]:
          return _capitalize_first_letter(word)
  
      # Some exceptions that would not produce good results with the set of
      # general_regular_expressions.
      regexes = current_app.config["CLASSIFIER_EXCEPTIONS"]
      if word in regexes:
          return _capitalize_first_letter(regexes[word])
  
      regexes = current_app.config["CLASSIFIER_UNCHANGE_REGULAR_EXPRESSIONS"]
      for regex in regexes:
          if regex.search(word) is not None:
              return _capitalize_first_letter(word)
  
      regexes = current_app.config["CLASSIFIER_GENERAL_REGULAR_EXPRESSIONS"]
      for regex, replacement in regexes:
          stemmed = regex.sub(replacement, word)
          if stemmed != word:
              return _capitalize_first_letter(stemmed)
  
      return _capitalize_first_letter(word + "s?")