assertNodeNotInGraphErrorMessage(
        assertThrows(IllegalArgumentException.class, () -> graph.predecessors(NODE_NOT_IN_GRAPH)));
  }

  @Test
  public void successors_noSuccessors() {
    addNode(N1);
    assertThat(graph.successors(N1)).isEmpty();
  }

  @Test
  public void successors_nodeNotInGraph() {
    assertNodeNotInGraphErrorMessage(

    assertNodeNotInGraphErrorMessage(
        assertThrows(
            IllegalArgumentException.class, () -> network.predecessors(NODE_NOT_IN_GRAPH)));
  }

  @Test
  public void successors_noSuccessors() {
    addNode(N1);
    assertThat(network.successors(N1)).isEmpty();
  }

  @Test
  public void successors_nodeNotInGraph() {
    assertNodeNotInGraphErrorMessage(