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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 _first_of_month(self, day_of_week):
 """ Modify to the first occurrence of a given day of the week in the current month. If no day_of_week is provided, modify to the first day of the month. Use the supplied consts to indicate the desired day_of_week, ex. DateTime.MONDAY. :type day_of_week: int :rtype: DateTime """

        dt = self.start_of("day")

        if day_of_week is None:
            return dt.set(day=1)

        month = calendar.monthcalendar(dt.year, dt.month)

        calendar_day = (day_of_week - 1) % 7

        if month[0][calendar_day] > 0:
            day_of_month = month[0][calendar_day]
        else:
            day_of_month = month[1][calendar_day]

        return dt.set(day=day_of_month)