assume().that(graphIsMutable()).isTrue();

    assertTrue(networkAsMutableNetwork.addNode(N1));
    assertThat(networkAsMutableNetwork.nodes()).contains(N1);
  }

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

    addNode(N1);
    ImmutableSet<Integer> nodes = ImmutableSet.copyOf(networkAsMutableNetwork.nodes());
    assertFalse(networkAsMutableNetwork.addNode(N1));