assertThat(graph.removeEdge(2, 1)).isNull();
    assertThat(graph.removeEdge(2, 3)).isEqualTo("valueC");
    assertThat(graph.removeEdge(2, 3)).isNull();
  }

  @Test
  public void removeEdge_directed_orderMismatch() {
    graph = ValueGraphBuilder.directed().build();
    graph.putEdgeValue(1, 2, "1->2");
    graph.putEdgeValue(2, 1, "2->1");
    IllegalArgumentException e =
        assertThrows(