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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 dict_of_lists(self):
 """ Returns a dictionary where each key is associated with a list of values. """

        result = {}
        for key, value in self._items:
            if key in result:
                result[key].append(value)
            else:
                result[key] = [value]
        return result