* Equivalence<@Nullable Number>. That can still produce wrappers of various types --
     * Wrapper<Number>, Wrapper<Integer>, Wrapper<@Nullable Integer>, etc. If we used just
     * Equivalence<? super T> below, no type could satisfy both that bound and T's own
     * bound. With this type, they have some overlap: in our example, Equivalence<Number>
     * and Equivalence<Object>.
     */