code: def test_add_nodes_from_string(self):
      self.graph.add_nodes_from(["a", "b", "c", "d"])
      self.assertListEqual(
          sorted(self.graph.nodes()), ["a", "b", "c", "d"]
      )
  
imports: from pgmpy.base import UndirectedGraph
  from pgmpy.tests import help_functions
  import unittest
label_desc: Use image node_id 6 for calling the TestUndirectedGraphCreation obj's underlying member method code with example usage: obj.test_add_nodes_from_string() without return types