}

  @Test
  public void edgesConnecting_parallelEdges_undirected() {
    assume().that(network.allowsParallelEdges()).isTrue();
    assume().that(network.isDirected()).isFalse();

    addEdge(N1, N2, E12);
    addEdge(N1, N2, E12_A);
    addEdge(N2, N1, E21);

    assertThat(network.edgesConnecting(N1, N2)).containsExactly(E12, E12_A, E21);

  }

  @Test
  public void edgesConnecting_parallelEdges_undirected() {
    assume().that(network.allowsParallelEdges()).isTrue();
    assume().that(network.isDirected()).isFalse();

    addEdge(N1, N2, E12);
    addEdge(N1, N2, E12_A);
    addEdge(N2, N1, E21);

    assertThat(network.edgesConnecting(N1, N2)).containsExactly(E12, E12_A, E21);

      }
    };
  }

  @Override
  @CheckForNull
  public 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));

  }

  @Override
  public Set<N> adjacentNodes(N node) {
    return nodeInvalidatableSet(checkedConnections(node).adjacentNodes(), node);
  }

  @Override
  public Set<E> edgesConnecting(N nodeU, N nodeV) {
    NetworkConnections<N, E> connectionsU = checkedConnections(nodeU);
    if (!allowsSelfLoops && nodeU == nodeV) { // just an optimization, only check reference equality
      return ImmutableSet.of();

    }

    @Override
    public Set<E> edgesConnecting(N nodeU, N nodeV) {
      return delegate().edgesConnecting(nodeV, nodeU); // transpose
    }

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

    @Override
    @CheckForNull

          + "so it cannot be reused to connect the following nodes: %s.";
  static final String MULTIPLE_EDGES_CONNECTING =
      "Cannot call edgeConnecting() when parallel edges exist between %s and %s. Consider calling "
          + "edgesConnecting() instead.";
  static final String PARALLEL_EDGES_NOT_ALLOWED =
      "Nodes %s and %s are already connected by a different edge. To construct a graph "

   *
   * <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()}).
   *