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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 last_day(self):
 """Return the last day of Yom Tov or Shabbat. This is useful for three-day holidays, for example: it will return the last in a string of Yom Tov + Shabbat. If this HDate is Shabbat followed by no Yom Tov, returns the Saturday. If this HDate is neither Yom Tov, nor Shabbat, this just returns itself. """

        day_iter = self
        while day_iter.next_day.is_yom_tov or day_iter.next_day.is_shabbat:
            day_iter = day_iter.next_day
        return day_iter