Constraint p = new IsEventFrom(DerivedEvent.class, o);
assertTrue(p.eval(good_ev));
assertTrue("p should eval to false for an event not from o",
!p.eval(wrong_source));
assertTrue("p should eval to false for an event of the wrong type",
!p.eval(wrong_type));
assertTrue("p should eval to false for an event of the wrong type " +
"and from the wrong source",
!p.eval(wrong_source_and_type));