using value_type = T::value_type;

using const_iterator = T::const_iterator;

constexpr T& self(){

return *static_cast<T*>(this);

}

constexpr const T& self() const{

return *static_cast<const T*>(this);

}

public:

constexpr value_type& front(){

return *self().begin();

}

constexpr const_iterator cbegin() const{

return self().begin();

}

constexpr const value_type& front() const{

return *cbegin();

}

constexpr value_type& back(){

return *std::prev(self().end());

}

constexpr const_iterator cend() const{

return self().end();

}

constexpr const value_type& back() const{

return *std::prev(cend());

}

constexpr T& operator[](std::size_t i) const{

return *std::next(begin(),i);

}