code: def parse_subprotocol_item(
      header: str, pos: int, header_name: str
  ) -> Tuple[Subprotocol, int]:
      
      item, pos = parse_token(header, pos, header_name)
      return cast(Subprotocol, item), pos
docs: Parse a subprotocol from ``header`` at the given position.
  
      Return the subprotocol value and the new position.
  
      Raise :exc:`~websockets.exceptions.InvalidHeaderFormat` on invalid inputs.