throw new IllegalArgumentException(String.format(MULTIPLE_EDGES_CONNECTING, nodeU, nodeV));
    }
  }

  @Override
  public @Nullable E edgeConnectingOrNull(EndpointPair<N> endpoints) {
    validateEndpoints(endpoints);
    return edgeConnectingOrNull(endpoints.nodeU(), endpoints.nodeV());
  }

  @Override
  public boolean hasEdgeConnecting(N nodeU, N nodeV) {
    checkNotNull(nodeU);

    }

    @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

   * <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));