code: def test_add_node_string(self):
      self.graph = DAG()
      self.graph.add_node("a")
      self.assertListEqual(list(self.graph.nodes()), ["a"])
      self.assertEqual(self.graph.latents, set())
  
      self.graph = DAG()
      self.graph.add_node("a", latent=True)
      self.assertListEqual(list(self.graph.nodes()), ["a"])
      self.assertEqual(self.graph.latents, set(["a"]))
  
imports: import unittest
  import networkx
  import numpy
  import pgmpy.tests.help_functions
  from pgmpy.base import DAG, PDAG
label_desc: Use image node_id 4 for calling the TestDAGCreation obj's underlying member method code with example usage: obj.test_add_node_string() without return types