docs: 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