code: def _etextno_to_uri_subdirectory(etextno):
      
      str_etextno = str(etextno).zfill(2)
      all_but_last_digit = list(str_etextno[:-1])
      subdir_part = "/".join(all_but_last_digit)
      subdir = "{}/{}".format(subdir_part, etextno)  # etextno not zfilled
      return subdir
docs: Returns the subdirectory that an etextno will be found in a gutenberg
      mirror. Generally, one finds the subdirectory by separating out each digit
      of the etext number, and uses it for a directory. The exception here is for
      etext numbers less than 10, which are prepended with a 0 for the directory
      traversal.
  
      >>> _etextno_to_uri_subdirectory(1)
      '0/1'
      >>> _etextno_to_uri_subdirectory(19)
      '1/19'
      >>> _etextno_to_uri_subdirectory(15453)
      '1/5/4/5/15453'