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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 add_transaction(self, transaction):
 """ Add a transaction. Arguments: transaction -- A transaction as an iterable object (eg. ['A', 'B']). """

        for item in transaction:
            if item not in self.__transaction_index_map:
                self.__items.append(item)
                self.__transaction_index_map[item] = set()
            self.__transaction_index_map[item].add(self.__num_transaction)
        self.__num_transaction += 1