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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:
 def reply_bytes(self, request):
 """Take a `Request` and return an OP_MSG message as bytes."""

        flags = struct.pack("<I", self._flags)
        payload_type = struct.pack("<b", 0)
        payload_data = bson.BSON.encode(self.doc)
        data = b''.join([flags, payload_type, payload_data])

        reply_id = random.randint(0, 1000000)
        response_to = request.request_id

        header = struct.pack(
            "<iiii", 16 + len(data), reply_id, response_to, OP_MSG)
        return header + data