docs: def prev_period(self):
          """
          The period before this range.
  
              >>> span = PeriodRange.from_date(date(2000, 1, 1), period="month")
              >>> span.prev_period()
              PeriodRange([datetime.date(1999, 12, 1),datetime.date(2000, 1, 1)))
  
          :return: A new :class:`~spans.types.PeriodRange` for the period
                   before this period
          """
  
          return self.from_date(self.prev(self.lower), period=self.period)