docs: def _apply(self, aggregator):
          """Returns a dataframe with the requested ColumnReductions.
          """
  
          reduced_dfs = []
          if self.include_fraction:
              n_df = self.numerator.apply_and_name(aggregator)
              d_df = self.denominator.apply_and_name(aggregator)
              reduced_dfs.extend([n_df[cn]/d_df[cd]
                                  for cn, cd in product(n_df.columns, d_df.columns)])
  
          if self.include_numerator:
              reduced_dfs.append(self.numerator.apply_and_name(aggregator))
  
          if self.include_denominator:
              reduced_dfs.append(self.denominator.apply_and_name(aggregator))
  
          return pd.concat(reduced_dfs, axis=1)