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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 _expand_batch(cls, batch):
 """Deserializes a Batch's header, and the header of its Transactions. """

        cls._parse_header(BatchHeader, batch)
        if 'transactions' in batch:
            batch['transactions'] = [
                cls._expand_transaction(t) for t in batch['transactions']]
        return batch