#include <compare>
#include <initializer_list>
 
namespace std {
  // class template deque
  template<class T, class Allocator = allocator<T>> class deque;
 
  template<class T, class Allocator>
    bool operator==(const deque<T, Allocator>& x, const deque<T, Allocator>& y);
  template<class T, class Allocator>
    __synth_three_way_result<T> operator<=>(const deque<T, Allocator>& x,
                                            const deque<T, Allocator>& y);
 
  template<class T, class Allocator>
    void swap(deque<T, Allocator>& x, deque<T, Allocator>& y)
      noexcept(noexcept(x.swap(y)));
 
  // erasure
  template<class T, class Allocator, class U>
    typename deque<T, Allocator>::size_type
      erase(deque<T, Allocator>& c, const U& value);
  template<class T, class Allocator, class Predicate>
    typename deque<T, Allocator>::size_type
      erase_if(deque<T, Allocator>& c, Predicate pred);
 
  namespace pmr {
    template<class T>
      using deque = std::deque<T, polymorphic_allocator<T>>;
  }
}

namespace std {
  template<class T, class Allocator = allocator<T>>
  class deque {
  public:
    // types
    using value_type             = T;
    using allocator_type         = Allocator;
    using pointer                = typename allocator_traits<Allocator>::pointer;
    using const_pointer          = typename allocator_traits<Allocator>::const_pointer;
    using reference              = value_type&;
    using const_reference        = const value_type&;
    using size_type              = /* implementation-defined */;
    using difference_type        = /* implementation-defined */;
    using iterator               = /* implementation-defined */;
    using const_iterator         = /* implementation-defined */;
    using reverse_iterator       = std::reverse_iterator<iterator>;
    using const_reverse_iterator = std::reverse_iterator<const_iterator>;
 
    // construct/copy/destroy
    deque() : deque(Allocator()) { }
    explicit deque(const Allocator&);
    explicit deque(size_type n, const Allocator& = Allocator());
    deque(size_type n, const T& value, const Allocator& = Allocator());
    template<class InputIt>
      deque(InputIt first, InputIt last, const Allocator& = Allocator());
    template<__container_compatible_range<T> R>
      deque(from_range_t, R&& rg, const Allocator& = Allocator());
    deque(const deque& x);
    deque(deque&&);
    deque(const deque&, const type_identity_t<Allocator>&);
    deque(deque&&, const type_identity_t<Allocator>&);
    deque(initializer_list<T>, const Allocator& = Allocator());
 
    ~deque();
    deque& operator=(const deque& x);
    deque& operator=(deque&& x)
      noexcept(allocator_traits<Allocator>::is_always_equal::value);
    deque& operator=(initializer_list<T>);
    template<class InputIt>
      void assign(InputIt first, InputIt last);
    template<__container_compatible_range<T> R>
      void assign_range(R&& rg);
    void assign(size_type n, const T& t);
    void assign(initializer_list<T>);
    allocator_type get_allocator() const noexcept;
 
    // iterators
    iterator               begin() noexcept;
    const_iterator         begin() const noexcept;
    iterator               end() noexcept;
    const_iterator         end() const noexcept;
    reverse_iterator       rbegin() noexcept;
    const_reverse_iterator rbegin() const noexcept;
    reverse_iterator       rend() noexcept;
    const_reverse_iterator rend() const noexcept;
 
    const_iterator         cbegin() const noexcept;
    const_iterator         cend() const noexcept;
    const_reverse_iterator crbegin() const noexcept;
    const_reverse_iterator crend() const noexcept;
 
    // capacity
    [[nodiscard]] bool empty() const noexcept;
    size_type size() const noexcept;
    size_type max_size() const noexcept;
    void      resize(size_type sz);
    void      resize(size_type sz, const T& c);
    void      shrink_to_fit();
 
    // element access
    reference       operator[](size_type n);
    const_reference operator[](size_type n) const;
    reference       at(size_type n);
    const_reference at(size_type n) const;
    reference       front();
    const_reference front() const;
    reference       back();
    const_reference back() const;
 
    // modifiers
    template<class... Args> reference emplace_front(Args&&... args);
    template<class... Args> reference emplace_back(Args&&... args);
    template<class... Args> iterator emplace(const_iterator position, Args&&... args);
 
    void push_front(const T& x);
    void push_front(T&& x);
    template<__container_compatible_range<T> R>
      void prepend_range(R&& rg);
    void push_back(const T& x);
    void push_back(T&& x);
    template<__container_compatible_range<T> R>
      void append_range(R&& rg);
 
    iterator insert(const_iterator position, const T& x);
    iterator insert(const_iterator position, T&& x);
    iterator insert(const_iterator position, size_type n, const T& x);
    template<class InputIt>
      iterator insert(const_iterator position, InputIt first, InputIt last);
    template<__container_compatible_range<T> R>
      iterator insert_range(const_iterator position, R&& rg);
    iterator insert(const_iterator position, initializer_list<T>);
 
    void pop_front();
    void pop_back();
 
    iterator erase(const_iterator position);
    iterator erase(const_iterator first, const_iterator last);
    void     swap(deque&)
      noexcept(allocator_traits<Allocator>::is_always_equal::value);
    void     clear() noexcept;
  };
 
  template<class InputIt, class Allocator = allocator<__iter_value_type<InputIt>>>
    deque(InputIt, InputIt, Allocator = Allocator())
      -> deque<__iter_value_type<InputIt>, Allocator>;
 
  template<ranges::input_range R, class Allocator = allocator<ranges::range_value_t<R>>>
    deque(from_range_t, R&&, Allocator = Allocator())
      -> deque<ranges::range_value_t<R>, Allocator>;
}