docs: def get_prefix_dir(archive):
      """
      Often, all files are in a single directory. If so, they'll all have
      the same prefix. Determine any such prefix.
      archive is a ZipFile
      """
      names = archive.namelist()
      shortest_name = sorted(names, key=len)[0]
      candidate_prefixes = [
          shortest_name[:length]
          for length in range(len(shortest_name), -1, -1)
      ]
      for prefix in candidate_prefixes:
          if all(name.startswith(prefix) for name in names):
              return prefix
      return ''