Class remove_if

example/tutorial/appendix_mpl.cpp:317–322

template <typename Sequence, typename Pred>
struct remove_if {
    using type = decltype(hana::filter(
        hana::to_tuple(Sequence{}),
        hana::compose(hana::not_, detail::mpl_predicate<Pred>)
    ));
};

template <typename Sequence, typename Pred>
struct unique {