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

  @Test
  public void incidentEdges_noIncidentEdges() {
    addNode(N1);
    assertThat(graph.incidentEdges(N1)).isEmpty();
  }

  @Test
  public void incidentEdges_nodeNotInGraph() {
    assertNodeNotInGraphErrorMessage(