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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 update_freshness(self, ledger_id, ts):
        '''
        Updates the time at which the ledger was updated.
        Should be called whenever a txn for the ledger is ordered.

        :param ledger_id: the ID of the ledgers a txn was ordered for
        :param ts: the current time
        :return: None
        '''
        if ledger_id in self._ledger_freshness:
            self._ledger_freshness[ledger_id].last_updated = ts