@Test
  public void nodes_oneNode() {
    addNode(N1);
    assertThat(graph.nodes()).containsExactly(N1);
  }

  @Test
  public void nodes_noNodes() {
    assertThat(graph.nodes()).isEmpty();
  }

  @Test
  public void adjacentNodes_oneEdge() {
    putEdge(N1, N2);
    assertThat(graph.adjacentNodes(N1)).containsExactly(N2);

  @Test
  public void nodes_oneNode() {
    addNode(N1);
    assertThat(network.nodes()).containsExactly(N1);
  }

  @Test
  public void nodes_noNodes() {
    assertThat(network.nodes()).isEmpty();
  }

  @Test
  public void edges_oneEdge() {
    addEdge(N1, N2, E12);
    assertThat(network.edges()).containsExactly(E12);
  }

  @Test