code: def subtrees(self):
      visit_list = []
      visit_postorder(self.root, visit_list)
      return visit_list
  
imports: import os
  from typing import List, Optional, Set
  import pandas
  from ludwig.datasets.dataset_config import DatasetConfig
  from ludwig.datasets.loaders.dataset_loader import DatasetLoader
label_desc: Use image node_id 4 for calling the SSTTree obj's underlying member method code with example usage: obj.subtrees() and returns: visit_list