addNode(N1);
    putEdge(N1, N1);
    assertThat(graphAsMutableGraph.removeNode(N1)).isTrue();
    assertThat(graph.nodes()).isEmpty();
  }

  @Test
  public void removeEdge_existingSelfLoopEdge() {
    assume().that(graphIsMutable()).isTrue();
    assume().that(graph.allowsSelfLoops()).isTrue();

    putEdge(N1, N1);
    assertThat(graphAsMutableGraph.removeEdge(N1, N1)).isTrue();

    assertThat(networkAsMutableNetwork.removeNode(N1)).isTrue();
    assertThat(network.nodes()).isEmpty();
    assertThat(network.edges()).doesNotContain(E11);
  }

  @Test
  public void removeEdge_existingSelfLoopEdge() {
    assume().that(graphIsMutable()).isTrue();
    assume().that(network.allowsSelfLoops()).isTrue();

    addEdge(N1, N1, E11);
    assertThat(networkAsMutableNetwork.removeEdge(E11)).isTrue();