public void outDegree_nodeNotInGraph() {
    assertNodeNotInGraphErrorMessage(
        assertThrows(IllegalArgumentException.class, () -> graph.outDegree(NODE_NOT_IN_GRAPH)));
  }

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

    assertThat(graphAsMutableGraph.addNode(N1)).isTrue();
    assertThat(graph.nodes()).contains(N1);
  }

  @Test

  public void successors_nodeNotInGraph() {
    assertNodeNotInGraphErrorMessage(
        assertThrows(IllegalArgumentException.class, () -> network.successors(NODE_NOT_IN_GRAPH)));
  }

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

    assertTrue(networkAsMutableNetwork.addNode(N1));
    assertThat(networkAsMutableNetwork.nodes()).contains(N1);
  }

  @Test