addEdge(N2, N1, E12);
    assertThat(network.outDegree(N1)).isEqualTo(3);
  }

  // Element Mutation

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

    // Adding nodes initially for safety (insulating from possible future
    // modifications to proxy methods)
    addNode(N1);
    addNode(N2);

  }

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

    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());