docs: def export(self, output_file_name):
          """exports engineered features
  
          Parameters
          ----------
          output_file_name: string
              String containing the path and file name of the desired output file
  
          Returns
          -------
          None
  
          """
          if self._best_estimator is None:
              raise ValueError('A model has not been optimized. Please call fit()'
                               ' first.')
  
          # Write print_model() to file
          with open(output_file_name, 'w') as output_file:
              output_file.write(self.print_model())
          # if decision tree, print tree into dot file
          if 'DecisionTree' in self.ml_type:
              export_graphviz(self._best_estimator,
                              out_file=output_file_name+'.dot',
                              feature_names = self.stacks_2_eqns(self._best_inds)
                              if self._best_inds else None,
                              class_names=['True','False'],
                              filled=False,impurity = True,rotate=True)