graph = ValueGraphBuilder.directed().build();
    graph.putEdgeValue(1, 2, "A");
    assertThat(graph.edgeValue(EndpointPair.ordered(1, 2))).hasValue("A");
  }

  @Test
  public void edgeValue_directed_backwards() {
    graph = ValueGraphBuilder.directed().build();
    graph.putEdgeValue(1, 2, "A");
    assertThat(graph.edgeValue(EndpointPair.ordered(2, 1))).isEmpty();
  }

  @Test