docs: def common_directory(paths):
      """Find the deepest common directory of a list of paths.
  
      :return: if no paths are provided, None is returned;
        if there is no common directory, '' is returned;
        otherwise the common directory with a trailing / is returned.
      """
      import posixpath
      def get_dir_with_slash(path):
          if path == b'' or path.endswith(b'/'):
              return path
          else:
              dirname, basename = posixpath.split(path)
              if dirname == b'':
                  return dirname
              else:
                  return dirname + b'/'
  
      if not paths:
          return None
      elif len(paths) == 1:
          return get_dir_with_slash(paths[0])
      else:
          common = common_path(paths[0], paths[1])
          for path in paths[2:]:
              common = common_path(common, path)
          return get_dir_with_slash(common)