docs: def _backward_word(text, pos):
      """
      Move pos back to the start of the current or previous word. Words
      are composed of alphanumeric characters (letters and digits).
      """
      while pos > 0 and not text[pos - 1].isalnum():
          pos -= 1
      while pos > 0 and text[pos - 1].isalnum():
          pos -= 1
      return text, pos