code: def test_domrel_png(self):
      # Graph taken from https://commons.wikipedia.org/wiki/File:Domrel.png
      edges = [(1, 2), (2, 3), (2, 4), (2, 6), (3, 5), (4, 5), (5, 2)]
      G = nx.DiGraph(edges)
      assert nx.dominance_frontiers(G, 1) == {
          1: set(),
          2: {2},
          3: {5},
          4: {5},
          5: {2},
          6: set(),
      }
      # Test postdominance.
      result = nx.dominance_frontiers(G.reverse(copy=False), 6)
      assert result == {
          1: set(),
          2: {2},
          3: {2},
          4: {2},
          5: {2},
          6: set(),
      }
  
imports: import pytest
  import networkx
label_desc: Use image node_id 8 for calling the TestDominanceFrontiers obj's underlying member method code with example usage: obj.test_domrel_png() without return types