<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _handle_websocket_headers(self, handler: WebSocketHandler) -> None: """Verifies all invariant- and required headers If a header is missing or have an incorrect value ValueError will be raised """

        fields = ("Host", "Sec-Websocket-Key", "Sec-Websocket-Version")
        if not all(map(lambda f: handler.request.headers.get(f), fields)):
            raise ValueError("Missing/Invalid WebSocket headers")