@Test
 public void testJoin6() {
   Stamp dExactNonNull = StampFactory.exactNonNull(getType(D.class));
   Stamp alwaysNull = StampFactory.alwaysNull();
   Stamp join = join(alwaysNull, dExactNonNull);
   Assert.assertFalse(join.isLegal());
   Assert.assertFalse(StampTool.isPointerNonNull(join));
   Assert.assertFalse(StampTool.isPointerAlwaysNull(join));
 }