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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 etag(self, etag):
 """ Sets the etag of this BulkResponse. etag :param etag: The etag of this BulkResponse. :type: str """

        if etag is None:
            raise ValueError("Invalid value for `etag`, must not be `None`")
        if etag is not None and not re.search('[A-Za-z0-9]{0,256}', etag):
            raise ValueError("Invalid value for `etag`, must be a follow pattern or equal to `/[A-Za-z0-9]{0,256}/`")

        self._etag = etag