#include <initializer_list>
 
namespace std {
 
    template <class T, size_t N> struct array;
 
    template <class T, size_t N>
        constexpr bool operator==(const array<T,N>& x, const array<T,N>& y);
    template <class T, size_t N>
        constexpr bool operator!=(const array<T,N>& x, const array<T,N>& y);
    template <class T, size_t N>
        constexpr bool operator<(const array<T,N>& x, const array<T,N>& y);
    template <class T, size_t N>
        constexpr bool operator>(const array<T,N>& x, const array<T,N>& y);
    template <class T, size_t N>
        constexpr bool operator<=(const array<T,N>& x, const array<T,N>& y);
    template <class T, size_t N>
        constexpr bool operator>=(const array<T,N>& x, const array<T,N>& y);
 
    template <class T, size_t N>
        constexpr void swap(array<T,N>& x, array<T,N>& y) noexcept(noexcept(x.swap(y)));
 
    template <class T> class tuple_size;
    template <size_t I, class T> class tuple_element;
    template <class T, size_t N> struct tuple_size<array<T, N> >;
    template <size_t I, class T, size_t N> struct tuple_element<I, array<T, N> >;
    template <size_t I, class T, size_t N>
    constexpr T& get(array<T, N>&) noexcept;
    template <size_t I, class T, size_t N>
    constexpr T&& get(array<T, N>&&) noexcept;
    template <size_t I, class T, size_t N>
    constexpr const T& get(const array<T, N>&) noexcept;
    template <size_t I, class T, size_t N>
    constexpr const T&& get(const array<T, N>&&) noexcept;
}

template <class T, size_t N >
struct array {
    // types:
    typedef T&                               reference;
    typedef const T&                         const_reference;
    typedef /*implementation-defined*/       iterator;
    typedef /*implementation-defined*/       const_iterator;
    typedef size_t                           size_type;
    typedef ptrdiff_t                        difference_type;
    typedef T                                value_type;
    typedef T*                               pointer;
    typedef const T*                         const_pointer;
    typedef reverse_iterator<iterator>       reverse_iterator;
    typedef reverse_iterator<const_iterator> const_reverse_iterator;
 
    T elems[N]; // exposition only
 
    // no explicit construct/copy/destroy for aggregate type
 
    constexpr void fill(const T& u);
    constexpr void swap(array<T, N>&) noexcept(is_nothrow_swappable_v<T>);
 
    // iterators:
    constexpr iterator                begin() noexcept;
    constexpr const_iterator          begin() const noexcept;
    constexpr iterator                end() noexcept;
    constexpr const_iterator          end() const noexcept;
 
    constexpr reverse_iterator        rbegin() noexcept;
    constexpr const_reverse_iterator  rbegin() const noexcept;
    constexpr reverse_iterator        rend() noexcept;
    constexpr const_reverse_iterator  rend() const noexcept;
 
    constexpr const_iterator          cbegin() const noexcept;
    constexpr const_iterator          cend() const noexcept;
    constexpr const_reverse_iterator  crbegin() const noexcept;
    constexpr const_reverse_iterator  crend() const noexcept;
 
    // capacity:
    constexpr size_type size() const noexcept;
    constexpr size_type max_size() const noexcept;
    constexpr bool      empty() const noexcept;
 
    // element access:
    constexpr reference       operator[](size_type n);
    constexpr const_reference operator[](size_type n) const;
    constexpr const_reference at(size_type n) const;
    constexpr reference       at(size_type n);
    constexpr reference       front();
    constexpr const_reference front() const;
    constexpr reference       back();
    constexpr const_reference back() const;
 
    constexpr T *       data() noexcept;
    constexpr const T * data() const noexcept;
};
 
template<class T, class... U>
  array(T, U...) -> array<T, 1 + sizeof...(U)>;