code: def get_reduced(self, column_reductions):
          
          for cr in column_reductions:
              if cr not in self.column_reductions:
                  raise ValueError("Column reduction %r is not known to this Aggregator!" % cr)
          return self.reduced_df[column_reductions]
docs: This function gets called by ColumnFunction._apply(). After a ColumnFunction
          has been passed to Aggregator's constructor, the ColumnFunction can use this function
          to request the populated, aggregated columns that correspond to its ColumnReductions.
  
          Args:
              column_reduction (list[ColumnReduction])
  
          Returns:
              pd.DataFrame: A dataframe, where the column names are ColumnReductions.