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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_week_start(day=None):
 """Returns the Monday of the given week."""

    day = add_timezone(day or datetime.date.today())
    days_since_monday = day.weekday()
    if days_since_monday != 0:
        day = day - relativedelta(days=days_since_monday)
    return day