code: def test_add_edge_string(self):
      self.G.add_edge("a", "b")
      self.assertCountEqual(list(self.G.nodes()), ["a", "b"])
      self.assertListEqual(list(self.G.edges()), [("a", "b")])
      self.assertEqual(self.G.dependent, "a")
      self.assertSetEqual(self.G.features, {"b"})
  
      self.G.add_nodes_from(["c", "d"])
      self.G.add_edge("a", "c")
      self.G.add_edge("a", "d")
      self.assertCountEqual(list(self.G.nodes()), ["a", "b", "c", "d"])
      self.assertCountEqual(
          list(self.G.edges()), [("a", "b"), ("a", "c"), ("a", "d")]
      )
      self.assertEqual(self.G.dependent, "a")
      self.assertSetEqual(self.G.features, {"b", "c", "d"})
  
      self.assertRaises(ValueError, self.G.add_edge, "b", "c")
      self.assertRaises(ValueError, self.G.add_edge, "d", "f")
      self.assertRaises(ValueError, self.G.add_edge, "e", "f")
      self.assertRaises(
          ValueError, self.G.add_edges_from, [("a", "e"), ("b", "f")]
      )
      self.assertRaises(ValueError, self.G.add_edges_from, [("b", "f")])
  
imports: import unittest
  import networkx
  import pandas
  import numpy
  from pgmpy.models import NaiveBayes
  from pgmpy.independencies import Independencies
label_desc: Use image node_id 9 for calling the TestBaseModelCreation obj's underlying member method code with example usage: obj.test_add_edge_string() without return types