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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 get_sunset_time(self, timeformat='unix'):
 """Returns the GMT time of sunset :param timeformat: the format for the time value. May be: '*unix*' (default) for UNIX time or '*iso*' for ISO8601-formatted string in the format ``YYYY-MM-DD HH:MM:SS+00`` :type timeformat: str :returns: an int or a str or None :raises: ValueError """

        if self._sunset_time is None:
            return None
        return timeformatutils.timeformat(self._sunset_time, timeformat)