* <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);

  /**

    }

    @Override
    public @Nullable E edgeConnectingOrNull(N nodeU, N nodeV) {
      return delegate().edgeConnectingOrNull(nodeV, nodeU); // transpose
    }

    @Override
    public @Nullable E edgeConnectingOrNull(EndpointPair<N> endpoints) {
      return delegate().edgeConnectingOrNull(transpose(endpoints));
    }

    @Override

    }

    @Override
    public @Nullable E edgeConnectingOrNull(N nodeU, N nodeV) {
      return delegate().edgeConnectingOrNull(nodeV, nodeU); // transpose
    }

    @Override
    public @Nullable E edgeConnectingOrNull(EndpointPair<N> endpoints) {
      return delegate().edgeConnectingOrNull(transpose(endpoints));
    }

    @Override

        switch (edgesConnecting.size()) {
          case 0:
            assertThat(network.edgeConnectingOrNull(node, otherNode)).isNull();
            assertThat(network.hasEdgeConnecting(node, otherNode)).isFalse();
            break;
          case 1:
            assertThat(network.edgeConnectingOrNull(node, otherNode))
                .isEqualTo(edgesConnecting.iterator().next());

    assertThat(transpose(transpose)).isSameInstanceAs(directedGraph);
    AbstractNetworkTest.validateNetwork(transpose);

    assertThat(transpose.edgesConnecting(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(EndpointPair.unordered(N1, N2));
            });
    assertThat(e).hasMessageThat().contains(ENDPOINTS_MISMATCH);
  }

  @Override
  @Test
  public void incidentNodes_oneEdge() {

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

  @Override
  @Test
  public void incidentNodes_oneEdge() {

    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);