* <p>In an undirected network, this is equal to {@code edgeConnectingOrNull(nodeV, nodeU)}.
   *
   * @throws IllegalArgumentException if there are multiple parallel edges connecting {@code nodeU}
   *     to {@code nodeV}
   * @throws IllegalArgumentException if {@code nodeU} or {@code nodeV} is not an element of this
   *     network
   * @since 23.0
   */
  @Nullable E edgeConnectingOrNull(N nodeU, N nodeV);

  /**

    AbstractNetworkTest.validateNetwork(transpose);

    assertThat(transpose.edgesConnecting(N1, N2)).isEmpty();
    assertThat(transpose.edgeConnecting(N1, N2)).isEmpty();
    assertThat(transpose.edgeConnectingOrNull(N1, N2)).isNull();

    for (Integer node : directedGraph.nodes()) {
      assertThat(directedGraph.inDegree(node)).isSameInstanceAs(transpose.outDegree(node));

    addEdge(N1, N2, E12);
    IllegalArgumentException e =
        assertThrows(
            IllegalArgumentException.class,
            () -> {
              String unused = network.edgeConnectingOrNull(ENDPOINTS_N1N2);
            });
    assertThat(e).hasMessageThat().contains(ENDPOINTS_MISMATCH);
  }

  @Test
  public void edgesConnecting_oneEdge() {
    addEdge(N1, N2, E12);