constexpr auto foldr_impl(F f, S s, ...) {
return foldr1(f, type<Xs>..., s);
}
constexpr auto foldr(F f, S s, Xs ...xs) {
return foldr1(f, xs..., s);
}
// foldr1(F, [x1, ..., xn]) = F(x1, F(x2, ..., F(xn-1,xn)...))
int foldr1(const fun2 F, const list L)
{
if (tl(L) == nil) return hd(L);
else
return F(hd(L), foldr1(F, tl(L)));
}
