static list_t append_helper(list_t first, list_t appended) {
if (list_isEmpty(first)) return appended;
return append_helper(list_rest(first),
list_make(list_first(first), appended));
}
list_t append(list_t first, list_t second) {
return append_helper(reverse(first), second);
}
static constexpr auto apply(Xs&& xs, X&& x) {
constexpr std::size_t N = decltype(hana::length(xs))::value;
return append_helper(static_cast<Xs&&>(xs), static_cast<X&&>(x),
std::make_index_sequence<N> {});
}
