}
  }

  @Override
  @Test
  public void edgesConnecting_checkReturnedSetMutability() {
    assume().that(graphIsMutable()).isTrue();

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

  }

  @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);

    assertThat(directedGraph.edges()).isEmpty();
    assertThat(directedGraph.addEdge(N1, N2, E12)).isTrue();
    assertThat(directedGraph.edgesConnecting(N1, N2)).isEqualTo(ImmutableSet.of(E12));
    assertThat(directedGraph.edgesConnecting(N2, N1)).isEmpty();

    // By default, parallel edges are not allowed.
    IllegalArgumentException e =

  }

  @Override
  @Test
  public void edgesConnecting_checkReturnedSetMutability() {
    addNode(N1);
    addNode(N2);
    Set<String> edgesConnecting = network.edgesConnecting(N1, N2);
    UnsupportedOperationException e =
        assertThrows(UnsupportedOperationException.class, () -> edgesConnecting.add(E23));
    addEdge(N1, N2, E12);
    assertThat(network.edgesConnecting(N1, N2)).containsExactlyElementsIn(edgesConnecting);

  }

  @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
  @Test
  public void edgesConnecting_checkReturnedSetMutability() {
    addNode(N1);
    addNode(N2);
    Set<String> edgesConnecting = network.edgesConnecting(N1, N2);
    UnsupportedOperationException e =
        assertThrows(UnsupportedOperationException.class, () -> edgesConnecting.add(E23));
    addEdge(N1, N2, E12);
    assertThat(network.edgesConnecting(N1, N2)).containsExactlyElementsIn(edgesConnecting);

    }
  }

  @Override
  @Test
  public void edgesConnecting_checkReturnedSetMutability() {
    assume().that(graphIsMutable()).isTrue();

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

    assertThat(directedGraph.edges()).isEmpty();
    assertThat(directedGraph.addEdge(N1, N2, E12)).isTrue();
    assertThat(directedGraph.edgesConnecting(N1, N2)).isEqualTo(ImmutableSet.of(E12));
    assertThat(directedGraph.edgesConnecting(N2, N1)).isEmpty();

    // By default, parallel edges are not allowed.
    IllegalArgumentException e =

      }
    };
  }

  @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<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