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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):
 """ Add an ETag option to the message. :param etag: the etag """

        if not isinstance(etag, list):
            etag = [etag]
        for e in etag:
            option = Option()
            option.number = defines.OptionRegistry.ETAG.number
            if not  isinstance(e, bytes):
                e = bytes(e, "utf-8")
            option.value = e
            self.add_option(option)