assertTrue(list.get(2) instanceof Book);
b = (Book) list.get(2);
assertEquals(2, b.getAttributes().size());
Document d = (Document) list.get(3);
assertEquals(0, d.getAttributes().size());
d = (Document) list.get(4);
assertEquals(0, d.getAttributes().size());
d = (Document) list.get(5);
assertEquals(0, d.getAttributes().size());
}