assertEquals(2, a.getChildren().size());
        assertTrue(a.getChildren().contains(b));
        assertTrue(a.getChildren().contains(d));
        assertEquals(2, d.getParents().size());
        assertTrue(d.getParents().contains(a));
        assertTrue(d.getParents().contains(c));
    }

    @Test
    public void testCycleDetection() throws Exception {
        Graph graph1 = new Graph();
        addEdge(graph1, "a", "b");

 *
 * <p>If you have your own graph implementation based around a custom node type {@code MyNode},
 * which has a method {@code getParents()} that retrieves its predecessors in a graph:
 *
 * {@snippet :
 * someGraphAlgorithm(startNode, MyNode::getParents);
 * }
 *
 * <p>If you have some other mechanism for returning the predecessors of a node, or one that doesn't

 *
 * <p>If you have your own graph implementation based around a custom node type {@code MyNode},
 * which has a method {@code getParents()} that retrieves its predecessors in a graph:
 *
 * {@snippet :
 * someGraphAlgorithm(startNode, MyNode::getParents);
 * }
 *
 * <p>If you have some other mechanism for returning the predecessors of a node, or one that doesn't

        }

        String getLabel() {
            return label;
        }

        List<Vertex> getChildren() {
            return children;
        }

        List<Vertex> getParents() {
            return parents;
        }
    }

        }

        String getLabel() {
            return label;
        }

        List<Vertex> getChildren() {
            return children;
        }

        List<Vertex> getParents() {
            return parents;
        }
    }

    static class CycleDetectedException extends RuntimeException {
        private final List<String> cycle;