@Test
    public void testAbstract() {
        assertTrue(Modifier.isAbstract(Emptyannotation.class.getModifiers()));
    }
Packs the given doc to Book
@param d doc, never null
@return a Book object or null if input doc isn't a Book
documentgen-test/src/test/java/com/yahoo/vespa/config/DocumentGenPluginTest.java