code: def test_backdoor_validation(self):
      self.assertTrue(
          self.inference.is_valid_backdoor_adjustment_set(
              "X", "Y", Z="C"
          )
      )
  
      # Z accepts str or set[str]
      self.assertTrue(
          self.inference_bd.is_valid_backdoor_adjustment_set(
              "X", "Y", Z="Z1"
          )
      )
      self.assertTrue(
          self.inference_bd2.is_valid_backdoor_adjustment_set(
              "X", "Y", Z={"Z1", "Z2"}
          )
      )
  
imports: import unittest
  import numpy
  import numpy.testing
  import pandas
  from pgmpy.factors.discrete import TabularCPD
  from pgmpy.inference.CausalInference import CausalInference
  from pgmpy.models import BayesianNetwork
label_desc: Use image node_id 3 for calling the TestCausalGraphMethods obj's underlying member method code with example usage: obj.test_backdoor_validation() without return types