Handle the Sec-WebSocket-Protocol HTTP request header.

        Return Sec-WebSocket-Protocol HTTP response header, which is the same
        as the selected subprotocol.

def process_subprotocol(
        self, headers: Headers, available_subprotocols: Optional[Sequence[Subprotocol]]
    ) -> Optional[Subprotocol]:
        """
        Handle the Sec-WebSocket-Protocol HTTP request header.

        Return Sec-WebSocket-Protocol HTTP response header, which is the same
        as the selected subprotocol.

        """
        subprotocol: Optional[Subprotocol] = None

        header_values = headers.get_all("Sec-WebSocket-Protocol")

        if header_values and available_subprotocols:

            parsed_header_values: List[Subprotocol] = sum(
                [parse_subprotocol(header_value) for header_value in header_values], []
            )

            subprotocol = self.select_subprotocol(
                parsed_header_values, available_subprotocols
            )

        return subprotocol