* otherwise, returns {@code defaultValue}.
   *
   * <p>In an undirected graph, this is equal to {@code edgeValueOrDefault(nodeV, nodeU,
   * defaultValue)}.
   *
   * @throws IllegalArgumentException if {@code nodeU} or {@code nodeV} is not an element of this
   *     graph
   */
  @CheckForNull
  V edgeValueOrDefault(N nodeU, N nodeV, @CheckForNull V defaultValue);

  /**

    assertThat(transpose).isEqualTo(expectedTranspose);
    assertThat(transpose(transpose)).isSameInstanceAs(directedGraph);
    AbstractGraphTest.validateGraph(transpose.asGraph());

    assertThat(transpose.edgeValueOrDefault(N1, N2, null)).isNull();
    for (Integer node : directedGraph.nodes()) {
      assertThat(directedGraph.inDegree(node)).isSameInstanceAs(transpose.outDegree(node));