assertThat(graphAsMutableGraph.addNode(N1)).isTrue();
    assertThat(graph.nodes()).contains(N1);
  }

  @Test
  public void addNode_existingNode() {
    assume().that(graphIsMutable()).isTrue();

    addNode(N1);
    ImmutableSet<Integer> nodes = ImmutableSet.copyOf(graph.nodes());
    assertThat(graphAsMutableGraph.addNode(N1)).isFalse();

    assertThat(graphAsMutableGraph.addNode(N1)).isTrue();
    assertThat(graph.nodes()).contains(N1);
  }

  @Test
  public void addNode_existingNode() {
    assume().that(graphIsMutable()).isTrue();

    addNode(N1);
    ImmutableSet<Integer> nodes = ImmutableSet.copyOf(graph.nodes());
    assertThat(graphAsMutableGraph.addNode(N1)).isFalse();

      assertThat(network.adjacentEdges(E12)).containsExactlyElementsIn(adjacentEdges);
    }
  }

  @Override
  @Test
  public void edgesConnecting_checkReturnedSetMutability() {
    addNode(N1);
    addNode(N2);
    Set<String> edgesConnecting = network.edgesConnecting(N1, N2);
    assertThrows(UnsupportedOperationException.class, () -> edgesConnecting.add(E23));
    addEdge(N1, N2, E12);

    mutableNetwork.addNode("A");
    ImmutableNetwork<String, Integer> immutableNetwork = ImmutableNetwork.copyOf(mutableNetwork);

    assertThat(immutableNetwork).isNotInstanceOf(MutableNetwork.class);
    assertThat(immutableNetwork).isEqualTo(mutableNetwork);

    mutableNetwork.addNode("B");
    assertThat(immutableNetwork).isNotEqualTo(mutableNetwork);
  }

  @Test

    mutableValueGraph.addNode("A");
    ImmutableValueGraph<String, Integer> immutableValueGraph =
        ImmutableValueGraph.copyOf(mutableValueGraph);

    assertThat(immutableValueGraph).isNotInstanceOf(MutableValueGraph.class);
    assertThat(immutableValueGraph).isEqualTo(mutableValueGraph);

    mutableValueGraph.addNode("B");
    assertThat(immutableValueGraph).isNotEqualTo(mutableValueGraph);

    directedGraph.putEdge(N2, N3);
    directedGraph.addNode(N4);

    // the above graph is its own transitive closure
    MutableGraph<Integer> expectedClosure = GraphBuilder.directed().allowsSelfLoops(true).build();
    expectedClosure.putEdge(N1, N2);
    expectedClosure.putEdge(N1, N3);
    expectedClosure.putEdge(N2, N3);
    expectedClosure.addNode(N4);

    assertThat(

      }
    };
  }

  private static ImmutableGraph<Character> createSingleRootGraph() {
    MutableGraph<Character> graph = GraphBuilder.directed().build();
    graph.addNode('a');
    return ImmutableGraph.copyOf(graph);
  }

  private static void assertEqualCharNodes(Iterable<Character> result, String expectedCharacters) {
    assertThat(result)