code: def register_unpack_format(name, extensions, function, extra_args=None,
                             description=''):
      
      if extra_args is None:
          extra_args = []
      _check_unpack_options(extensions, function, extra_args)
      _UNPACK_FORMATS[name] = extensions, function, extra_args, description
docs: Registers an unpack format.
  
      `name` is the name of the format. `extensions` is a list of extensions
      corresponding to the format.
  
      `function` is the callable that will be
      used to unpack archives. The callable will receive archives to unpack.
      If it's unable to handle an archive, it needs to raise a ReadError
      exception.
  
      If provided, `extra_args` is a sequence of
      (name, value) tuples that will be passed as arguments to the callable.
      description can be provided to describe the format, and will be returned
      by the get_unpack_formats() function.