Set<E> edgesConnecting = edgesConnecting(nodeU, nodeV);
    switch (edgesConnecting.size()) {
      case 0:
        return null;
      case 1:
        return edgesConnecting.iterator().next();
      default:
        throw new IllegalArgumentException(String.format(MULTIPLE_EDGES_CONNECTING, nodeU, nodeV));
    }
  }

  @Override
  public @Nullable E edgeConnectingOrNull(EndpointPair<N> endpoints) {

   *
   * <p>In an undirected network, this is equal to {@code edgesConnecting(nodeV, nodeU)}.
   *
   * <p>The resulting set of edges will be parallel (i.e. have equal {@link
   * #incidentNodes(Object)}). If this network does not {@link #allowsParallelEdges() allow parallel
   * edges}, the resulting set will contain at most one edge (equivalent to {@code
   * edgeConnecting(nodeU, nodeV).asSet()}).
   *

    assertThat(transpose(transpose)).isSameInstanceAs(directedGraph);
    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()) {

    addNode(N1);
    addNode(N2);
    Set<String> edgesConnecting = network.edgesConnecting(N1, N2);
    assertThrows(UnsupportedOperationException.class, () -> edgesConnecting.add(E23));
    addEdge(N1, N2, E12);
    assertThat(network.edgesConnecting(N1, N2)).containsExactlyElementsIn(edgesConnecting);
  }

  @Override
  @Test
  public void inEdges_checkReturnedSetMutability() {

  }

  @Override
  public @Nullable E edgeConnectingOrNull(N nodeU, N nodeV) {
    Set<E> edgesConnecting = edgesConnecting(nodeU, nodeV);
    switch (edgesConnecting.size()) {
      case 0:
        return null;
      case 1:
        return edgesConnecting.iterator().next();
      default:
        throw new IllegalArgumentException(String.format(MULTIPLE_EDGES_CONNECTING, nodeU, nodeV));
    }