@Theory
 public void shouldHold(@ForAll @From(Ctor.class) P p) {
   assertNotNull(p.z());
   assertNotNull(p.f());
   assertNotNull(p.b());
 }
 @Theory
 public void shouldHold(@ForAll @From(Ctor.class) P p) {
   assertTrue(p.box().marked());
   assertTrue(p.box().contents().marked());
   assertEquals(2, p.box().contents().i());
 }