code: def test_active_trail_nodes(self):
      self.network.add_edges_from(
          [
              (("a", 0), ("a", 1)),
              (("a", 0), ("b", 1)),
              (("b", 0), ("b", 1)),
          ]
      )
  
      active_trail = self.network.active_trail_nodes(("a", 0))
      self.assertListEqual(
          sorted(active_trail.get(("a", 0))),
          [("a", 0), ("a", 1), ("b", 1)],
      )
  
      active_trail = self.network.active_trail_nodes(
          ("a", 0), observed=[("b", 1)]
      )
      self.assertListEqual(
          sorted(active_trail.get(("a", 0))),
          [("a", 0), ("a", 1), ("b", 0)],
      )
  
imports: import unittest
  import numpy
  import numpy.testing
  import pandas
  import pgmpy.tests.help_functions
  from pgmpy.factors.discrete import TabularCPD
  from pgmpy.inference import CausalInference, DBNInference, VariableElimination
  from pgmpy.models import BayesianNetwork
  from pgmpy.models import DynamicBayesianNetwork
label_desc: Use image node_id 16 for calling the TestDynamicBayesianNetworkMethods obj's underlying member method code with example usage: obj.test_active_trail_nodes() without return types