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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 aggregate_by_index(self, function, level=0):
 """ Aggregrate data in each record, grouping by index values. For each unique value of the index, applies a function to the group indexed by that value. Returns a Series indexed by those unique values. For the result to be a valid Series object, the aggregating function should return a simple numeric type. Also allows selection of levels within a multi-index. See select_by_index for more info on indices and multi-indices. Parameters function : function Aggregating function to map to Series values. Should take a list or ndarray as input and return a simple numeric value. level : list of ints, optional, default=0 Specifies the levels of the multi-index to use when determining unique index values. If only a single level is desired, can be an int. """

        result = self._map_by_index(function, level=level)
        return result.map(lambda v: array(v), index=result.index)