code: def test_add_single_edge(self):
      self.graph.add_edge("a", "phi1")
      self.assertListEqual(sorted(self.graph.nodes()), ["a", "phi1"])
      self.assertListEqual(
          hf.recursive_sorted(self.graph.edges()), [["a", "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 6 for calling the TestFactorGraphCreation obj's underlying member method code with example usage: obj.test_add_single_edge() without return types