docs: def readf(nb_file, fmt=None):
      """Read a notebook from the file with given name"""
      if nb_file == '-':
          text = sys.stdin.read()
          fmt = fmt or divine_format(text)
          return reads(text, fmt)
  
      _, ext = os.path.splitext(nb_file)
      fmt = copy(fmt or {})
      fmt.update({'extension': ext})
      with io.open(nb_file, encoding='utf-8') as stream:
          return read(stream, fmt, as_version=4)