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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 timestamp(self, posix: bool = True, **kwargs) -> Union[str, int]: """Generate random timestamp. :param posix: POSIX time. :param kwargs: Kwargs for :meth:`~Datetime.datetime()`. :return: Timestamp. """

        stamp = self.datetime(**kwargs)

        if posix:
            return timegm(stamp.utctimetuple())

        return stamp.strftime('%Y-%m-%dT%H:%M:%SZ')