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);
    assertThat(graph.adjacentNodes(N2)).containsExactly(N1);
  }

  @Test

    assertEdgeNotInGraphErrorMessage(
        assertThrows(
            IllegalArgumentException.class, () -> network.incidentNodes(EDGE_NOT_IN_GRAPH)));
  }

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

  @Test