code: def to_string(self):
      r_list = []
      for i in range(self.num_children):
          if isinstance(self.children[i], Tree):
              r_list.append("( " + self.children[i].to_string() + " )")
          else:
              r_list.append(" " + str(self.children[i]) + " ")
      return "".join(r_list)
  
imports: from operator import itemgetter
  import numpy
label_desc: Use image node_id 4 for calling the Tree obj's underlying member method code with example usage: obj.to_string() and returns: str