IllegalArgumentException.class, () -> graph.edgeValue(EndpointPair.unordered(2, 1)));
    assertThat(e).hasMessageThat().contains(ENDPOINTS_MISMATCH);
  }

  @Test
  public void edgeValue_undirected_correct() {
    graph = ValueGraphBuilder.undirected().build();
    graph.putEdgeValue(1, 2, "A");
    assertThat(graph.edgeValue(EndpointPair.unordered(1, 2))).hasValue("A");
  }

  @Test