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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 rollback(self):
 """Rolls back pending mutations. Keep in mind that NoTransactionBatch splits all mutations into smaller batches and commit them as soon as mutation buffer reaches maximum length. That's why rollback method will only roll back pending mutations from the buffer, but won't be able to rollback already committed mutations. """

    try:
      if self._cur_batch:
        self._cur_batch.rollback()
    except ValueError:
      # ignore "Batch must be in progress to rollback" error
      pass
    self._cur_batch = None
    self._num_mutations = 0