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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 transactions(self, dt=None):
 """Retrieve the dict-form of all of the transactions in a given bar or for the whole simulation. Parameters dt : pd.Timestamp or None, optional The particular datetime to look up transactions for. If not passed, or None is explicitly passed, all of the transactions will be returned. Returns ------- transactions : list[dict] The transaction information. """

        if dt is None:
            # flatten the by-day transactions
            return [
                txn
                for by_day in itervalues(self._processed_transactions)
                for txn in by_day
            ]

        return self._processed_transactions.get(dt, [])