@Test
  public void adjacentEdges_noAdjacentEdges() {
    addEdge(N1, N2, E12);
    addEdge(N3, N4, E34);
    assertThat(network.adjacentEdges(E12)).isEmpty();
  }

  @Test
  public void adjacentEdges_edgeNotInGraph() {
    assertEdgeNotInGraphErrorMessage(
        assertThrows(
            IllegalArgumentException.class, () -> network.adjacentEdges(EDGE_NOT_IN_GRAPH)));
  }

  @Test