code: def append_stem(to_transform, word):
      
      if is_sequence(to_transform):
          return [append_stem(f, word) for f in to_transform]
      elif is_string(to_transform):
          (base, ext) = splitext_plus(to_transform)
          return "".join([base, word, ext])
      else:
          raise ValueError("append_stem takes a single filename as a string or "
                           "a list of filenames to transform.")
docs: renames a filename or list of filenames with 'word' appended to the stem
      of each one:
      example: append_stem("/path/to/test.sam", "_filtered") ->
      "/path/to/test_filtered.sam"