っていうか
sign n | n > 0 = 1
| n == 0 = 0
| n < 0 = -1↓↓↓↓↓↓↓↓↓↓
template<class Integer, class = disable_if<less_equal<Iteger, zero> >::type>
struct sign : public int_<1>{ };
template<class Integer>
struct sign<Integer, enable_if<equal_to<Integer, zero> >::type>
: public int_<0>{ };
template<class Integer>
struct sign<Integer, eable_if<less<Integer, zero> >::type>
: public int_<-1>{ };・・・ダメ？
