graph.putEdgeValue(2, 3, "valueC");

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

  @Test
  public void removeEdge_undirected() {

    graph.putEdgeValue(2, 3, "valueC");

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

  @Test
  public void removeEdge_undirected() {

  }

  @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(networkAsMutableNetwork.edgesConnecting(N1, N2)).isEmpty();
  }

  @Test

    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
  public void removeNode_existingNodeWithSelfLoopEdge() {

        if (cycle != null) {
            // remove edge which introduced cycle
            removeEdge(from, to);
            throw new CycleDetectedException(
                    "Edge between '" + from.label + "' and '" + to.label + "' introduces to cycle in the graph", cycle);
        }
    }

    void removeEdge(Vertex from, Vertex to) {
        from.children.remove(to);
        to.parents.remove(from);
    }

    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();
    assertThat(graph.successors(N1)).isEmpty();
    assertThat(graph.predecessors(N2)).isEmpty();
  }

  @Test
  public void removeEdge_oneOfMany() {

    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
  public void removeNode_existingNodeWithSelfLoopEdge() {

      Collections.shuffle(edgeList, gen);
      int numEdgesToRemove = gen.nextInt(NUM_EDGES);
      for (int i = 0; i < numEdgesToRemove; ++i) {
        Object edge = edgeList.get(i);
        assertThat(network.removeEdge(edge)).isTrue();
      }

      assertThat(network.nodes()).hasSize(NUM_NODES);
      assertThat(network.edges()).hasSize(NUM_EDGES - numEdgesToRemove);
      AbstractNetworkTest.validateNetwork(network);

      Collections.shuffle(edgeList, gen);
      int numEdgesToRemove = gen.nextInt(NUM_EDGES);
      for (int i = 0; i < numEdgesToRemove; ++i) {
        Object edge = edgeList.get(i);
        assertThat(network.removeEdge(edge)).isTrue();
      }

      assertThat(network.nodes()).hasSize(NUM_NODES);
      assertThat(network.edges()).hasSize(NUM_EDGES - numEdgesToRemove);
      AbstractNetworkTest.validateNetwork(network);

  public void removeEdge_existingSelfLoopEdge() {
    assume().that(graphIsMutable()).isTrue();
    assume().that(network.allowsSelfLoops()).isTrue();

    addEdge(N1, N1, E11);
    assertThat(networkAsMutableNetwork.removeEdge(E11)).isTrue();
    assertThat(network.edges()).doesNotContain(E11);
    assertThat(network.edgesConnecting(N1, N1)).isEmpty();
  }