// Network with no edges, given disconnected nodes
    addNode(N1);
    addNode(N2);
    assertThat(network.edges()).isEmpty();
  }

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

  @Test