addEdge(N1, N2, E12);
    assertThat(network.incidentEdges(N2)).containsExactly(E12);
    assertThat(network.incidentEdges(N1)).containsExactly(E12);
  }

  @Test
  public void incidentEdges_isolatedNode() {
    addNode(N1);
    assertThat(network.incidentEdges(N1)).isEmpty();
  }

  @Test
  public void incidentEdges_nodeNotInGraph() {
    assertNodeNotInGraphErrorMessage(
        assertThrows(