<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def sub_working_days(self, day, delta, extra_working_days=None, extra_holidays=None, keep_datetime=False):
 """ Substract `delta` working days to the date. This method is a shortcut / helper. Users may want to use either:: cal.add_working_days(my_date, -7) cal.sub_working_days(my_date, 7) The other parameters are to be used exactly as in the ``add_working_days`` method. A negative ``delta`` argument will be converted into its absolute value. Hence, the two following calls are equivalent:: cal.sub_working_days(my_date, -7) cal.sub_working_days(my_date, 7) As in ``add_working_days()`` you can set the parameter ``keep_datetime`` to ``True`` to make sure that if your ``day`` argument is a ``datetime``, the returned date will also be a ``datetime`` object. """

        delta = abs(delta)
        return self.add_working_days(
            day, -delta,
            extra_working_days, extra_holidays, keep_datetime=keep_datetime)