code: def test_add_single_node(self):
      self.graph.add_node("phi1")
      self.assertEqual(list(self.graph.nodes()), ["phi1"])
  
imports: import numpy
  import unittest
  from pgmpy.factors.discrete import DiscreteFactor
  from pgmpy.models import FactorGraph
  from pgmpy.models import MarkovNetwork
  from pgmpy.models import JunctionTree
  from pgmpy.tests import help_functions
label_desc: Use image node_id 4 for calling the TestFactorGraphCreation obj's underlying member method code with example usage: obj.test_add_single_node() without return types