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#include <initializer_list> // mostly freestandingnamespace std{// constantsinlineconstexpr size_t dynamic_extent= numeric_limits<size_t>::max(); template<class T>  concept/*integral-constant-like*/=// exposition only    is_integral_v<decltype(T::value)>&&!is_same_v<bool, remove_const_t<decltype(T::value)>>&&    convertible_to<T, decltype(T::value)>&&    equality_comparable_with<T, decltype(T::value)>&&    bool_constant<T()== T::value>::value&&    bool_constant<static_cast<decltype(T::value)>(T())== T::value>::value; template<class T>constexpr size_t/*maybe-static-ext*/= dynamic_extent;// exposition onlytemplate</*integral-constant-like*/ T>constexpr size_t/*maybe-static-ext*/<T>={ T::value}; // class template spantemplate<class ElementType, size_t Extent= dynamic_extent>class span;// partially freestanding template<class ElementType, size_t Extent>constexprboolranges::enable_view<span<ElementType, Extent>>=true;template<class ElementType, size_t Extent>constexprboolranges::enable_borrowed_range<span<ElementType, Extent>>=true; // views of object representationtemplate<class ElementType, size_t Extent>  span<const byte,       Extent== dynamic_extent? dynamic_extent: sizeof(ElementType)* Extent>  as_bytes(span<ElementType, Extent> s)noexcept; template<class ElementType, size_t Extent>  span<byte, Extent== dynamic_extent? dynamic_extent: sizeof(ElementType)* Extent>  as_writable_bytes(span<ElementType, Extent> s)noexcept;}

namespace std{template<class ElementType, size_t Extent= dynamic_extent>class span{public:// constants and typesusing element_type= ElementType;using value_type= remove_cv_t<ElementType>;using size_type= size_t;using difference_type= ptrdiff_t;using pointer= element_type*;using const_pointer=const element_type*;using reference= element_type&;using const_reference=const element_type&;using iterator=/* implementation-defined */;using const_iterator=std::const_iterator<iterator>;using reverse_iterator=std::reverse_iterator<iterator>;using const_reverse_iterator=std::const_iterator<reverse_iterator>;staticconstexpr size_type extent= Extent; // constructors, copy, and assignmentconstexpr span()noexcept;template<class It>constexprexplicit(extent!= dynamic_extent) span(It first, size_type count);template<class It,class End>constexprexplicit(extent!= dynamic_extent) span(It first, End last);template<size_t N>constexpr span(type_identity_t<element_type>(&arr)[N])noexcept;template<class T, size_t N>constexpr span(array<T, N>& arr)noexcept;template<class T, size_t N>constexpr span(const array<T, N>& arr)noexcept;template<class R>constexprexplicit(extent!= dynamic_extent) span(R&& r);constexprexplicit(extent!= dynamic_extent)      span(std::initializer_list<value_type> il);constexpr span(const span& other)noexcept=default;template<class OtherElementType, size_t OtherExtent>constexprexplicit(/* see description */)      span(const span<OtherElementType, OtherExtent>& s)noexcept; constexpr span& operator=(const span& other)noexcept=default; // subviewstemplate<size_t Count>constexpr span<element_type, Count> first()const;template<size_t Count>constexpr span<element_type, Count> last()const;template<size_t Offset, size_t Count= dynamic_extent>constexpr span<element_type,/* see description */> subspan()const; constexpr span<element_type, dynamic_extent> first(size_type count)const;constexpr span<element_type, dynamic_extent> last(size_type count)const;constexpr span<element_type, dynamic_extent> subspan(      size_type offset,      size_type count= dynamic_extent)const; // observersconstexpr size_type size()constnoexcept;constexpr size_type size_bytes()constnoexcept;constexprbool empty()constnoexcept; // element accessconstexpr reference operator[](size_type idx)const;constexpr reference at(size_type idx)const;// freestanding-deletedconstexpr reference front()const;constexpr reference back()const;constexpr pointer data()constnoexcept; // iterator supportconstexpr iterator begin()constnoexcept;constexpr iterator end()constnoexcept;constexpr const_iterator cbegin()constnoexcept{return begin();}constexpr const_iterator cend()constnoexcept{return end();}constexpr reverse_iterator rbegin()constnoexcept;constexpr reverse_iterator rend()constnoexcept;constexpr const_reverse_iterator crbegin()constnoexcept{return rbegin();}constexpr const_reverse_iterator crend()constnoexcept{return rend();} private:    pointer/*data_*/;// exposition only    size_type/*size_*/;// exposition only}; template<class It,class EndOrSize>  span(It, EndOrSize)-> span<remove_reference_t<iter_reference_t<It>>,/*maybe-static-ext*/<EndOrSize>>;template<class T, size_t N>  span(T(&)[N])-> span<T, N>;template<class T, size_t N>  span(array<T, N>&)-> span<T, N>;template<class T, size_t N>  span(const array<T, N>&)-> span<const T, N>;template<class R>  span(R&&)-> span<remove_reference_t<ranges::range_reference_t<R>>>;}