code: def traverse(self, level=""):
      print(level + str(self))
      this_level = level
      if self.left:
          if isinstance(self.left, GBDTNode):
              self.left.traverse(level=this_level + " ")
      if self.right:
          if isinstance(self.right, GBDTNode):
              self.right.traverse(level=this_level + " ")
  
imports: from __future__ import print_function
  from __future__ import division
  from __future__ import absolute_import
  import turicreate
  import unittest
  import json
  import random
  import numpy
  import pytest
label_desc: Use image node_id 5 for calling the GBDTNode obj's underlying member method code with example usage: obj.traverse(level) without return types