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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 previous(self):
 """ Returns the previous batch for the batched sequence or `None`, if this batch is already the first batch. :rtype: :class:`Batch` instance or `None`. """

        if self.start - self.size < 0:
            result = None
        else:
            result = Batch(self.start - self.size, self.size, self.total_size)
        return result