public static Stream<ItemSet> apply(FPTree tree) {
        FPGrowth growth = new FPGrowth(tree);
        return StreamSupport.stream(growth.spliterator(), false);
    }
@Test
    public void testSinglePath() {
        System.out.println("single path");

        FPTree tree = FPTree.of(1, itemsets2);
        assertEquals(15, FPGrowth.apply(tree).count());
    }