code: def to_list(self, tgt_vocab):
      r_list = []
      for i in range(self.num_children):
          if isinstance(self.children[i], type(self)):
              r_list.append(tgt_vocab.get_symbol_idx("("))
              cl = self.children[i].to_list(tgt_vocab)
              for k in range(len(cl)):
                  r_list.append(cl[k])
              r_list.append(tgt_vocab.get_symbol_idx(")"))
          else:
              r_list.append(self.children[i])
      return r_list
  
imports: from operator import itemgetter
  import numpy
label_desc: Use image node_id 6 for calling the Tree obj's underlying member method code with example usage: obj.to_list(tgt_vocab) and returns: r_list