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(

   *
   * @return the value previously associated with the edge connecting {@code nodeU} to {@code
   *     nodeV}, or null if there was no such edge.
   */
  @CanIgnoreReturnValue
  @CheckForNull
  V removeEdge(N nodeU, N nodeV);

  /**
   * Removes the edge connecting {@code endpoints}, if it is present.
   *
   * <p>If this graph is directed, {@code endpoints} must be ordered.
   *

    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(

    assertThat(n2InEdges).isEmpty();
    assertThat(n2OutEdges).isEmpty();
  }

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

    addEdge(N1, N2, E12);
    assertTrue(networkAsMutableNetwork.removeEdge(E12));
    assertFalse(networkAsMutableNetwork.removeEdge(E12));
    assertThat(networkAsMutableNetwork.edges()).doesNotContain(E12);

    assertThat(n2InEdges).isEmpty();
    assertThat(n2OutEdges).isEmpty();
  }

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

    addEdge(N1, N2, E12);
    assertTrue(networkAsMutableNetwork.removeEdge(E12));
    assertFalse(networkAsMutableNetwork.removeEdge(E12));
    assertThat(networkAsMutableNetwork.edges()).doesNotContain(E12);

    assertThat(graphAsMutableGraph.putEdge(N1, N1)).isFalse();
  }

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

    putEdge(N1, N2);
    putEdge(N2, N1);

    assertThat(graphAsMutableGraph.removeEdge(N1, N2)).isTrue();
    assertThat(graph.successors(N1)).isEmpty();
    assertThat(graph.predecessors(N1)).containsExactly(N2);

  }

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

    putEdge(N1, N2);
    putEdge(N2, N1); // no-op

    assertThat(graphAsMutableGraph.removeEdge(N1, N2)).isTrue();
    assertThat(graph.adjacentNodes(N1)).isEmpty();
    assertThat(graph.edges()).isEmpty();
    assertThat(graphAsMutableGraph.removeEdge(N2, N1)).isFalse();
  }

  @Test

    return backingValueGraph.removeNode(node);
  }

  @Override
  public boolean removeEdge(N nodeU, N nodeV) {
    return backingValueGraph.removeEdge(nodeU, nodeV) != null;
  }

  @Override
  public boolean removeEdge(EndpointPair<N> endpoints) {
    validateEndpoints(endpoints);
    return removeEdge(endpoints.nodeU(), endpoints.nodeV());
  }

  }

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

    putEdge(N1, N2);
    putEdge(N2, N1); // no-op

    assertThat(graphAsMutableGraph.removeEdge(N1, N2)).isTrue();
    assertThat(graph.adjacentNodes(N1)).isEmpty();
    assertThat(graph.edges()).isEmpty();
    assertThat(graphAsMutableGraph.removeEdge(N2, N1)).isFalse();
  }

  @Test

  }

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

    putEdge(N1, N2);
    assertThat(graph.successors(N1)).containsExactly(N2);
    assertThat(graph.predecessors(N2)).containsExactly(N1);
    assertThat(graphAsMutableGraph.removeEdge(N1, N2)).isTrue();
    assertThat(graphAsMutableGraph.removeEdge(N1, N2)).isFalse();