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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 transaction_id(self):
 """ Transaction ID for Transbank, a secure random int between 0 and 999999999. """

        if not self._transaction_id:
            self._transaction_id = random.randint(0, 10000000000 - 1)
        return self._transaction_id