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);
    }

    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() {

  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();
  }

  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();
  }

  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();
  }

  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();
  }