docs: def reply_bytes(self, request):
          """Take a `Request` and return an OP_REPLY message as bytes."""
          flags = struct.pack("<i", self._flags)
          cursor_id = struct.pack("<q", self._cursor_id)
          starting_from = struct.pack("<i", self._starting_from)
          number_returned = struct.pack("<i", len(self._docs))
          reply_id = random.randint(0, 1000000)
          response_to = request.request_id
  
          data = b''.join([flags, cursor_id, starting_from, number_returned])
          data += b''.join([bson.BSON.encode(doc) for doc in self._docs])
  
          message = struct.pack("<i", 16 + len(data))
          message += struct.pack("<i", reply_id)
          message += struct.pack("<i", response_to)
          message += struct.pack("<i", OP_REPLY)
          return message + data