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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 async def start_server(self, sock: anyio.abc.SocketStream, filter=None):
 # pylint: disable=W0622 """Start a server WS connection on this socket. Filter: an async callable that gets passed the initial Request. It may return an AcceptConnection message, a bool, or a string (the subprotocol to use). Returns: the Request message. """

        assert self._scope is None
        self._scope = True
        self._sock = sock
        self._connection = WSConnection(ConnectionType.SERVER)

        try:
            event = await self._next_event()
            if not isinstance(event, Request):
                raise ConnectionError("Failed to establish a connection",
                                      event)
            msg = None
            if filter is not None:
                msg = await filter(event)
                if not msg:
                    msg = RejectConnection()
                elif msg is True:
                    msg = None
                elif isinstance(msg, str):
                    msg = AcceptConnection(subprotocol=msg)
            if not msg:
                msg = AcceptConnection(subprotocol=event.subprotocols[0])
            data = self._connection.send(msg)
            await self._sock.send_all(data)
            if not isinstance(msg, AcceptConnection):
                raise ConnectionError("Not accepted", msg)
        finally:
            self._scope = None