I guess this means contravariance, which commonly comes from having a method that accepts an argument of the same type. Consider Comparable in Java. Maybe you want to say Animal implements Comparable<Animal>. But that would mean that any Animal is comparable to any other Animal - the compiler would let you compare a Cat to a Dog. Whereas really what you want to do is reuse the implementation of Animal, but Cats are only comparable to other Cats, and Dogs are only comparable to other Dogs - so Cat and Dog should inherit from Animal, but their types won't actually be subtypes of Animal. (There's no way to express this constraint in Java).