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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 get(self, txn_id):
 """Returns the TransactionReceipt Args: txn_id (str):
 the id of the transaction for which the receipt should be retrieved. Returns: TransactionReceipt: The receipt for the given transaction id. Raises: KeyError: if the transaction id is unknown. """

        if txn_id not in self._receipt_db:
            raise KeyError('Unknown transaction id {}'.format(txn_id))

        txn_receipt_bytes = self._receipt_db[txn_id]
        txn_receipt = TransactionReceipt()
        txn_receipt.ParseFromString(txn_receipt_bytes)
        return txn_receipt