@Test
    public void testInheritanceOfGeneratedTypes() {
        assertEquals(Music3.class.getSuperclass(), Document.class);
        assertEquals(Music4.class.getSuperclass(), Document.class);
        assertEquals(Music.class.getSuperclass(), Common.class);
        assertEquals(Book.class.getSuperclass(), Common.class);
    }
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