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 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP___FLAT_MAP_FLAT_MULTIMAP_H
 #define _LIBCPP___FLAT_MAP_FLAT_MULTIMAP_H
 
 #include <__algorithm/lexicographical_compare_three_way.h>
 #include <__algorithm/min.h>
 #include <__algorithm/ranges_equal.h>
 #include <__algorithm/ranges_equal_range.h>
 #include <__algorithm/ranges_inplace_merge.h>
 #include <__algorithm/ranges_is_sorted.h>
 #include <__algorithm/ranges_lower_bound.h>
 #include <__algorithm/ranges_partition_point.h>
 #include <__algorithm/ranges_sort.h>
 #include <__algorithm/ranges_unique.h>
 #include <__algorithm/ranges_upper_bound.h>
 #include <__algorithm/remove_if.h>
 #include <__assert>
 #include <__compare/synth_three_way.h>
 #include <__concepts/convertible_to.h>
 #include <__concepts/swappable.h>
 #include <__config>
 #include <__cstddef/byte.h>
 #include <__cstddef/ptrdiff_t.h>
 #include <__flat_map/key_value_iterator.h>
 #include <__flat_map/sorted_equivalent.h>
 #include <__flat_map/utils.h>
 #include <__functional/invoke.h>
 #include <__functional/is_transparent.h>
 #include <__functional/operations.h>
 #include <__fwd/vector.h>
 #include <__iterator/concepts.h>
 #include <__iterator/distance.h>
 #include <__iterator/iterator_traits.h>
 #include <__iterator/ranges_iterator_traits.h>
 #include <__iterator/reverse_iterator.h>
 #include <__memory/allocator_traits.h>
 #include <__memory/uses_allocator.h>
 #include <__memory/uses_allocator_construction.h>
 #include <__ranges/access.h>
 #include <__ranges/concepts.h>
 #include <__ranges/container_compatible_range.h>
 #include <__ranges/drop_view.h>
 #include <__ranges/from_range.h>
 #include <__ranges/ref_view.h>
 #include <__ranges/size.h>
 #include <__ranges/subrange.h>
 #include <__ranges/zip_view.h>
 #include <__type_traits/conjunction.h>
 #include <__type_traits/container_traits.h>
 #include <__type_traits/invoke.h>
 #include <__type_traits/is_allocator.h>
 #include <__type_traits/is_nothrow_constructible.h>
 #include <__type_traits/is_same.h>
 #include <__type_traits/maybe_const.h>
 #include <__utility/exception_guard.h>
 #include <__utility/move.h>
 #include <__utility/pair.h>
 #include <__utility/scope_guard.h>
 #include <__vector/vector.h>
 #include <initializer_list>
 #include <stdexcept>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
 
 #if _LIBCPP_STD_VER >= 23
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _Key,
           class _Tp,
           class _Compare         = less<_Key>,
           class _KeyContainer    = vector<_Key>,
           class _MappedContainer = vector<_Tp>>
 class flat_multimap {
   template <class, class, class, class, class>
   friend class flat_multimap;
 
   static_assert(is_same_v<_Key, typename _KeyContainer::value_type>);
   static_assert(is_same_v<_Tp, typename _MappedContainer::value_type>);
   static_assert(!is_same_v<_KeyContainer, std::vector<bool>>, "vector<bool> is not a sequence container");
   static_assert(!is_same_v<_MappedContainer, std::vector<bool>>, "vector<bool> is not a sequence container");
 
   template <bool _Const>
   using __iterator _LIBCPP_NODEBUG = __key_value_iterator<flat_multimap, _KeyContainer, _MappedContainer, _Const>;
 
 public:
   // types
   using key_type               = _Key;
   using mapped_type            = _Tp;
   using value_type             = pair<key_type, mapped_type>;
   using key_compare            = __type_identity_t<_Compare>;
   using reference              = pair<const key_type&, mapped_type&>;
   using const_reference        = pair<const key_type&, const mapped_type&>;
   using size_type              = size_t;
   using difference_type        = ptrdiff_t;
   using iterator               = __iterator<false>; // see [container.requirements]
   using const_iterator         = __iterator<true>;  // see [container.requirements]
   using reverse_iterator       = std::reverse_iterator<iterator>;
   using const_reverse_iterator = std::reverse_iterator<const_iterator>;
   using key_container_type     = _KeyContainer;
   using mapped_container_type  = _MappedContainer;
 
   class value_compare {
   private:
     _LIBCPP_NO_UNIQUE_ADDRESS key_compare __comp_;
     _LIBCPP_HIDE_FROM_ABI value_compare(key_compare __c) : __comp_(__c) {}
     friend flat_multimap;
 
   public:
     _LIBCPP_HIDE_FROM_ABI bool operator()(const_reference __x, const_reference __y) const {
       return __comp_(__x.first, __y.first);
     }
   };
 
   struct containers {
     key_container_type keys;
     mapped_container_type values;
   };
 
 private:
   template <class _Allocator>
   _LIBCPP_HIDE_FROM_ABI static constexpr bool __allocator_ctor_constraint =
       _And<uses_allocator<key_container_type, _Allocator>, uses_allocator<mapped_container_type, _Allocator>>::value;
 
   _LIBCPP_HIDE_FROM_ABI static constexpr bool __is_compare_transparent = __is_transparent_v<_Compare>;
 
 public:
   // [flat.map.cons], construct/copy/destroy
   _LIBCPP_HIDE_FROM_ABI flat_multimap() noexcept(
       is_nothrow_default_constructible_v<_KeyContainer> && is_nothrow_default_constructible_v<_MappedContainer> &&
       is_nothrow_default_constructible_v<_Compare>)
       : __containers_(), __compare_() {}
 
   _LIBCPP_HIDE_FROM_ABI flat_multimap(const flat_multimap&) = default;
 
   // The copy/move constructors are not specified in the spec, which means they should be defaulted.
   // However, the move constructor can potentially leave a moved-from object in an inconsistent
   // state if an exception is thrown.
   _LIBCPP_HIDE_FROM_ABI flat_multimap(flat_multimap&& __other) noexcept(
       is_nothrow_move_constructible_v<_KeyContainer> && is_nothrow_move_constructible_v<_MappedContainer> &&
       is_nothrow_move_constructible_v<_Compare>)
 #  if _LIBCPP_HAS_EXCEPTIONS
       try
 #  endif // _LIBCPP_HAS_EXCEPTIONS
       : __containers_(std::move(__other.__containers_)), __compare_(std::move(__other.__compare_)) {
     __other.clear();
 #  if _LIBCPP_HAS_EXCEPTIONS
   } catch (...) {
     __other.clear();
     // gcc does not like the `throw` keyword in a conditionally noexcept function
     if constexpr (!(is_nothrow_move_constructible_v<_KeyContainer> &&
                     is_nothrow_move_constructible_v<_MappedContainer> && is_nothrow_move_constructible_v<_Compare>)) {
       throw;
     }
 #  endif // _LIBCPP_HAS_EXCEPTIONS
   }
 
   template <class _Allocator>
     requires __allocator_ctor_constraint<_Allocator>
   _LIBCPP_HIDE_FROM_ABI flat_multimap(const flat_multimap& __other, const _Allocator& __alloc)
       : flat_multimap(__ctor_uses_allocator_tag{},
                       __alloc,
                       __other.__containers_.keys,
                       __other.__containers_.values,
                       __other.__compare_) {}
 
   template <class _Allocator>
     requires __allocator_ctor_constraint<_Allocator>
   _LIBCPP_HIDE_FROM_ABI flat_multimap(flat_multimap&& __other, const _Allocator& __alloc)
 #  if _LIBCPP_HAS_EXCEPTIONS
       try
 #  endif // _LIBCPP_HAS_EXCEPTIONS
       : flat_multimap(__ctor_uses_allocator_tag{},
                       __alloc,
                       std::move(__other.__containers_.keys),
                       std::move(__other.__containers_.values),
                       std::move(__other.__compare_)) {
     __other.clear();
 #  if _LIBCPP_HAS_EXCEPTIONS
   } catch (...) {
     __other.clear();
     throw;
 #  endif // _LIBCPP_HAS_EXCEPTIONS
   }
 
   _LIBCPP_HIDE_FROM_ABI flat_multimap(
       key_container_type __key_cont, mapped_container_type __mapped_cont, const key_compare& __comp = key_compare())
       : __containers_{.keys = std::move(__key_cont), .values = std::move(__mapped_cont)}, __compare_(__comp) {
     _LIBCPP_ASSERT_VALID_INPUT_RANGE(__containers_.keys.size() == __containers_.values.size(),
                                      "flat_multimap keys and mapped containers have different size");
     __sort();
   }
 
   template <class _Allocator>
     requires __allocator_ctor_constraint<_Allocator>
   _LIBCPP_HIDE_FROM_ABI flat_multimap(
       const key_container_type& __key_cont, const mapped_container_type& __mapped_cont, const _Allocator& __alloc)
       : flat_multimap(__ctor_uses_allocator_tag{}, __alloc, __key_cont, __mapped_cont) {
     _LIBCPP_ASSERT_VALID_INPUT_RANGE(__containers_.keys.size() == __containers_.values.size(),
                                      "flat_multimap keys and mapped containers have different size");
     __sort();
   }
 
   template <class _Allocator>
     requires __allocator_ctor_constraint<_Allocator>
   _LIBCPP_HIDE_FROM_ABI
   flat_multimap(const key_container_type& __key_cont,
                 const mapped_container_type& __mapped_cont,
                 const key_compare& __comp,
                 const _Allocator& __alloc)
       : flat_multimap(__ctor_uses_allocator_tag{}, __alloc, __key_cont, __mapped_cont, __comp) {
     _LIBCPP_ASSERT_VALID_INPUT_RANGE(__containers_.keys.size() == __containers_.values.size(),
                                      "flat_multimap keys and mapped containers have different size");
     __sort();
   }
 
   _LIBCPP_HIDE_FROM_ABI
   flat_multimap(sorted_equivalent_t,
                 key_container_type __key_cont,
                 mapped_container_type __mapped_cont,
                 const key_compare& __comp = key_compare())
       : __containers_{.keys = std::move(__key_cont), .values = std::move(__mapped_cont)}, __compare_(__comp) {
     _LIBCPP_ASSERT_VALID_INPUT_RANGE(__containers_.keys.size() == __containers_.values.size(),
                                      "flat_multimap keys and mapped containers have different size");
     _LIBCPP_ASSERT_SEMANTIC_REQUIREMENT(__is_sorted(__containers_.keys), "Key container is not sorted");
   }
 
   template <class _Allocator>
     requires __allocator_ctor_constraint<_Allocator>
   _LIBCPP_HIDE_FROM_ABI
   flat_multimap(sorted_equivalent_t,
                 const key_container_type& __key_cont,
                 const mapped_container_type& __mapped_cont,
                 const _Allocator& __alloc)
       : flat_multimap(__ctor_uses_allocator_tag{}, __alloc, __key_cont, __mapped_cont) {
     _LIBCPP_ASSERT_VALID_INPUT_RANGE(__containers_.keys.size() == __containers_.values.size(),
                                      "flat_multimap keys and mapped containers have different size");
     _LIBCPP_ASSERT_SEMANTIC_REQUIREMENT(__is_sorted(__containers_.keys), "Key container is not sorted");
   }
 
   template <class _Allocator>
     requires __allocator_ctor_constraint<_Allocator>
   _LIBCPP_HIDE_FROM_ABI
   flat_multimap(sorted_equivalent_t,
                 const key_container_type& __key_cont,
                 const mapped_container_type& __mapped_cont,
                 const key_compare& __comp,
                 const _Allocator& __alloc)
       : flat_multimap(__ctor_uses_allocator_tag{}, __alloc, __key_cont, __mapped_cont, __comp) {
     _LIBCPP_ASSERT_VALID_INPUT_RANGE(__containers_.keys.size() == __containers_.values.size(),
                                      "flat_multimap keys and mapped containers have different size");
     _LIBCPP_ASSERT_SEMANTIC_REQUIREMENT(__is_sorted(__containers_.keys), "Key container is not sorted");
   }
 
   _LIBCPP_HIDE_FROM_ABI explicit flat_multimap(const key_compare& __comp) : __containers_(), __compare_(__comp) {}
 
   template <class _Allocator>
     requires __allocator_ctor_constraint<_Allocator>
   _LIBCPP_HIDE_FROM_ABI flat_multimap(const key_compare& __comp, const _Allocator& __alloc)
       : flat_multimap(__ctor_uses_allocator_empty_tag{}, __alloc, __comp) {}
 
   template <class _Allocator>
     requires __allocator_ctor_constraint<_Allocator>
   _LIBCPP_HIDE_FROM_ABI explicit flat_multimap(const _Allocator& __alloc)
       : flat_multimap(__ctor_uses_allocator_empty_tag{}, __alloc) {}
 
   template <class _InputIterator>
     requires __has_input_iterator_category<_InputIterator>::value
   _LIBCPP_HIDE_FROM_ABI
   flat_multimap(_InputIterator __first, _InputIterator __last, const key_compare& __comp = key_compare())
       : __containers_(), __compare_(__comp) {
     insert(__first, __last);
   }
 
   template <class _InputIterator, class _Allocator>
     requires(__has_input_iterator_category<_InputIterator>::value && __allocator_ctor_constraint<_Allocator>)
   _LIBCPP_HIDE_FROM_ABI
   flat_multimap(_InputIterator __first, _InputIterator __last, const key_compare& __comp, const _Allocator& __alloc)
       : flat_multimap(__ctor_uses_allocator_empty_tag{}, __alloc, __comp) {
     insert(__first, __last);
   }
 
   template <class _InputIterator, class _Allocator>
     requires(__has_input_iterator_category<_InputIterator>::value && __allocator_ctor_constraint<_Allocator>)
   _LIBCPP_HIDE_FROM_ABI flat_multimap(_InputIterator __first, _InputIterator __last, const _Allocator& __alloc)
       : flat_multimap(__ctor_uses_allocator_empty_tag{}, __alloc) {
     insert(__first, __last);
   }
 
   template <_ContainerCompatibleRange<value_type> _Range>
   _LIBCPP_HIDE_FROM_ABI flat_multimap(from_range_t __fr, _Range&& __rg)
       : flat_multimap(__fr, std::forward<_Range>(__rg), key_compare()) {}
 
   template <_ContainerCompatibleRange<value_type> _Range, class _Allocator>
     requires __allocator_ctor_constraint<_Allocator>
   _LIBCPP_HIDE_FROM_ABI flat_multimap(from_range_t, _Range&& __rg, const _Allocator& __alloc)
       : flat_multimap(__ctor_uses_allocator_empty_tag{}, __alloc) {
     insert_range(std::forward<_Range>(__rg));
   }
 
   template <_ContainerCompatibleRange<value_type> _Range>
   _LIBCPP_HIDE_FROM_ABI flat_multimap(from_range_t, _Range&& __rg, const key_compare& __comp) : flat_multimap(__comp) {
     insert_range(std::forward<_Range>(__rg));
   }
 
   template <_ContainerCompatibleRange<value_type> _Range, class _Allocator>
     requires __allocator_ctor_constraint<_Allocator>
   _LIBCPP_HIDE_FROM_ABI flat_multimap(from_range_t, _Range&& __rg, const key_compare& __comp, const _Allocator& __alloc)
       : flat_multimap(__ctor_uses_allocator_empty_tag{}, __alloc, __comp) {
     insert_range(std::forward<_Range>(__rg));
   }
 
   template <class _InputIterator>
     requires __has_input_iterator_category<_InputIterator>::value
   _LIBCPP_HIDE_FROM_ABI flat_multimap(
       sorted_equivalent_t, _InputIterator __first, _InputIterator __last, const key_compare& __comp = key_compare())
       : __containers_(), __compare_(__comp) {
     insert(sorted_equivalent, __first, __last);
   }
   template <class _InputIterator, class _Allocator>
     requires(__has_input_iterator_category<_InputIterator>::value && __allocator_ctor_constraint<_Allocator>)
   _LIBCPP_HIDE_FROM_ABI
   flat_multimap(sorted_equivalent_t,
                 _InputIterator __first,
                 _InputIterator __last,
                 const key_compare& __comp,
                 const _Allocator& __alloc)
       : flat_multimap(__ctor_uses_allocator_empty_tag{}, __alloc, __comp) {
     insert(sorted_equivalent, __first, __last);
   }
 
   template <class _InputIterator, class _Allocator>
     requires(__has_input_iterator_category<_InputIterator>::value && __allocator_ctor_constraint<_Allocator>)
   _LIBCPP_HIDE_FROM_ABI
   flat_multimap(sorted_equivalent_t, _InputIterator __first, _InputIterator __last, const _Allocator& __alloc)
       : flat_multimap(__ctor_uses_allocator_empty_tag{}, __alloc) {
     insert(sorted_equivalent, __first, __last);
   }
 
   _LIBCPP_HIDE_FROM_ABI flat_multimap(initializer_list<value_type> __il, const key_compare& __comp = key_compare())
       : flat_multimap(__il.begin(), __il.end(), __comp) {}
 
   template <class _Allocator>
     requires __allocator_ctor_constraint<_Allocator>
   _LIBCPP_HIDE_FROM_ABI
   flat_multimap(initializer_list<value_type> __il, const key_compare& __comp, const _Allocator& __alloc)
       : flat_multimap(__il.begin(), __il.end(), __comp, __alloc) {}
 
   template <class _Allocator>
     requires __allocator_ctor_constraint<_Allocator>
   _LIBCPP_HIDE_FROM_ABI flat_multimap(initializer_list<value_type> __il, const _Allocator& __alloc)
       : flat_multimap(__il.begin(), __il.end(), __alloc) {}
 
   _LIBCPP_HIDE_FROM_ABI
   flat_multimap(sorted_equivalent_t, initializer_list<value_type> __il, const key_compare& __comp = key_compare())
       : flat_multimap(sorted_equivalent, __il.begin(), __il.end(), __comp) {}
 
   template <class _Allocator>
     requires __allocator_ctor_constraint<_Allocator>
   _LIBCPP_HIDE_FROM_ABI flat_multimap(
       sorted_equivalent_t, initializer_list<value_type> __il, const key_compare& __comp, const _Allocator& __alloc)
       : flat_multimap(sorted_equivalent, __il.begin(), __il.end(), __comp, __alloc) {}
 
   template <class _Allocator>
     requires __allocator_ctor_constraint<_Allocator>
   _LIBCPP_HIDE_FROM_ABI flat_multimap(sorted_equivalent_t, initializer_list<value_type> __il, const _Allocator& __alloc)
       : flat_multimap(sorted_equivalent, __il.begin(), __il.end(), __alloc) {}
 
   _LIBCPP_HIDE_FROM_ABI flat_multimap& operator=(initializer_list<value_type> __il) {
     clear();
     insert(__il);
     return *this;
   }
 
   // copy/move assignment are not specified in the spec (defaulted)
   // but move assignment can potentially leave moved from object in an inconsistent
   // state if an exception is thrown
   _LIBCPP_HIDE_FROM_ABI flat_multimap& operator=(const flat_multimap&) = default;
 
   _LIBCPP_HIDE_FROM_ABI flat_multimap& operator=(flat_multimap&& __other) noexcept(
       is_nothrow_move_assignable_v<_KeyContainer> && is_nothrow_move_assignable_v<_MappedContainer> &&
       is_nothrow_move_assignable_v<_Compare>) {
     auto __clear_other_guard = std::__make_scope_guard([&]() noexcept { __other.clear() /* noexcept */; });
     auto __clear_self_guard  = std::__make_exception_guard([&]() noexcept { clear() /* noexcept */; });
     __containers_            = std::move(__other.__containers_);
     __compare_               = std::move(__other.__compare_);
     __clear_self_guard.__complete();
     return *this;
   }
 
   // iterators
   _LIBCPP_HIDE_FROM_ABI iterator begin() noexcept {
     return iterator(__containers_.keys.begin(), __containers_.values.begin());
   }
 
   _LIBCPP_HIDE_FROM_ABI const_iterator begin() const noexcept {
     return const_iterator(__containers_.keys.begin(), __containers_.values.begin());
   }
 
   _LIBCPP_HIDE_FROM_ABI iterator end() noexcept {
     return iterator(__containers_.keys.end(), __containers_.values.end());
   }
 
   _LIBCPP_HIDE_FROM_ABI const_iterator end() const noexcept {
     return const_iterator(__containers_.keys.end(), __containers_.values.end());
   }
 
   _LIBCPP_HIDE_FROM_ABI reverse_iterator rbegin() noexcept { return reverse_iterator(end()); }
   _LIBCPP_HIDE_FROM_ABI const_reverse_iterator rbegin() const noexcept { return const_reverse_iterator(end()); }
   _LIBCPP_HIDE_FROM_ABI reverse_iterator rend() noexcept { return reverse_iterator(begin()); }
   _LIBCPP_HIDE_FROM_ABI const_reverse_iterator rend() const noexcept { return const_reverse_iterator(begin()); }
 
   _LIBCPP_HIDE_FROM_ABI const_iterator cbegin() const noexcept { return begin(); }
   _LIBCPP_HIDE_FROM_ABI const_iterator cend() const noexcept { return end(); }
   _LIBCPP_HIDE_FROM_ABI const_reverse_iterator crbegin() const noexcept { return const_reverse_iterator(end()); }
   _LIBCPP_HIDE_FROM_ABI const_reverse_iterator crend() const noexcept { return const_reverse_iterator(begin()); }
 
   // [flat.map.capacity], capacity
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI bool empty() const noexcept { return __containers_.keys.empty(); }
 
   _LIBCPP_HIDE_FROM_ABI size_type size() const noexcept { return __containers_.keys.size(); }
 
   _LIBCPP_HIDE_FROM_ABI size_type max_size() const noexcept {
     return std::min<size_type>(__containers_.keys.max_size(), __containers_.values.max_size());
   }
 
   // [flat.map.modifiers], modifiers
   template <class... _Args>
     requires is_constructible_v<pair<key_type, mapped_type>, _Args...> && is_move_constructible_v<key_type> &&
              is_move_constructible_v<mapped_type>
   _LIBCPP_HIDE_FROM_ABI iterator emplace(_Args&&... __args) {
     std::pair<key_type, mapped_type> __pair(std::forward<_Args>(__args)...);
     auto __key_it    = ranges::upper_bound(__containers_.keys, __pair.first, __compare_);
     auto __mapped_it = __corresponding_mapped_it(*this, __key_it);
 
     return __flat_map_utils::__emplace_exact_pos(
         *this, std::move(__key_it), std::move(__mapped_it), std::move(__pair.first), std::move(__pair.second));
   }
 
   template <class... _Args>
     requires is_constructible_v<pair<key_type, mapped_type>, _Args...>
   _LIBCPP_HIDE_FROM_ABI iterator emplace_hint(const_iterator __hint, _Args&&... __args) {
     std::pair<key_type, mapped_type> __pair(std::forward<_Args>(__args)...);
 
     auto __prev_larger  = __hint != cbegin() && __compare_(__pair.first, (__hint - 1)->first);
     auto __next_smaller = __hint != cend() && __compare_(__hint->first, __pair.first);
 
     auto __hint_distance = __hint.__key_iter_ - __containers_.keys.cbegin();
     auto __key_iter      = __containers_.keys.begin() + __hint_distance;
     auto __mapped_iter   = __containers_.values.begin() + __hint_distance;
 
     if (!__prev_larger && !__next_smaller) [[likely]] {
       // hint correct, just use exact hint iterators
     } else if (__prev_larger && !__next_smaller) {
       // the hint position is more to the right than the key should have been.
       // we want to emplace the element to a position as right as possible
       // e.g. Insert new element "2" in the following range
       // 1, 1, 2, 2, 2, 3, 4, 6
       //                   ^
       //                   |
       //                  hint
       // We want to insert "2" after the last existing "2"
       __key_iter    = ranges::upper_bound(__containers_.keys.begin(), __key_iter, __pair.first, __compare_);
       __mapped_iter = __corresponding_mapped_it(*this, __key_iter);
     } else {
       _LIBCPP_ASSERT_INTERNAL(!__prev_larger && __next_smaller, "this means that the multimap is not sorted");
 
       // the hint position is more to the left than the key should have been.
       // we want to emplace the element to a position as left as possible
       //  1, 1, 2, 2, 2, 3, 4, 6
       //  ^
       //  |
       // hint
       // We want to insert "2" before the first existing "2"
       __key_iter    = ranges::lower_bound(__key_iter, __containers_.keys.end(), __pair.first, __compare_);
       __mapped_iter = __corresponding_mapped_it(*this, __key_iter);
     }
     return __flat_map_utils::__emplace_exact_pos(
         *this, __key_iter, __mapped_iter, std::move(__pair.first), std::move(__pair.second));
   }
 
   _LIBCPP_HIDE_FROM_ABI iterator insert(const value_type& __x) { return emplace(__x); }
 
   _LIBCPP_HIDE_FROM_ABI iterator insert(value_type&& __x) { return emplace(std::move(__x)); }
 
   _LIBCPP_HIDE_FROM_ABI iterator insert(const_iterator __hint, const value_type& __x) {
     return emplace_hint(__hint, __x);
   }
 
   _LIBCPP_HIDE_FROM_ABI iterator insert(const_iterator __hint, value_type&& __x) {
     return emplace_hint(__hint, std::move(__x));
   }
 
   template <class _PairLike>
     requires is_constructible_v<pair<key_type, mapped_type>, _PairLike>
   _LIBCPP_HIDE_FROM_ABI iterator insert(_PairLike&& __x) {
     return emplace(std::forward<_PairLike>(__x));
   }
 
   template <class _PairLike>
     requires is_constructible_v<pair<key_type, mapped_type>, _PairLike>
   _LIBCPP_HIDE_FROM_ABI iterator insert(const_iterator __hint, _PairLike&& __x) {
     return emplace_hint(__hint, std::forward<_PairLike>(__x));
   }
 
   template <class _InputIterator>
     requires __has_input_iterator_category<_InputIterator>::value
   _LIBCPP_HIDE_FROM_ABI void insert(_InputIterator __first, _InputIterator __last) {
     if constexpr (sized_sentinel_for<_InputIterator, _InputIterator>) {
       __reserve(__last - __first);
     }
     __append_sort_merge</*WasSorted = */ false>(std::move(__first), std::move(__last));
   }
 
   template <class _InputIterator>
     requires __has_input_iterator_category<_InputIterator>::value
   _LIBCPP_HIDE_FROM_ABI void insert(sorted_equivalent_t, _InputIterator __first, _InputIterator __last) {
     if constexpr (sized_sentinel_for<_InputIterator, _InputIterator>) {
       __reserve(__last - __first);
     }
 
     __append_sort_merge</*WasSorted = */ true>(std::move(__first), std::move(__last));
   }
 
   template <_ContainerCompatibleRange<value_type> _Range>
   _LIBCPP_HIDE_FROM_ABI void insert_range(_Range&& __range) {
     if constexpr (ranges::sized_range<_Range>) {
       __reserve(ranges::size(__range));
     }
 
     __append_sort_merge</*WasSorted = */ false>(ranges::begin(__range), ranges::end(__range));
   }
 
   _LIBCPP_HIDE_FROM_ABI void insert(initializer_list<value_type> __il) { insert(__il.begin(), __il.end()); }
 
   _LIBCPP_HIDE_FROM_ABI void insert(sorted_equivalent_t, initializer_list<value_type> __il) {
     insert(sorted_equivalent, __il.begin(), __il.end());
   }
 
   _LIBCPP_HIDE_FROM_ABI containers extract() && {
     auto __guard = std::__make_scope_guard([&]() noexcept { clear() /* noexcept */; });
     auto __ret   = std::move(__containers_);
     return __ret;
   }
 
   _LIBCPP_HIDE_FROM_ABI void replace(key_container_type&& __key_cont, mapped_container_type&& __mapped_cont) {
     _LIBCPP_ASSERT_VALID_INPUT_RANGE(
         __key_cont.size() == __mapped_cont.size(), "flat_multimap keys and mapped containers have different size");
 
     _LIBCPP_ASSERT_SEMANTIC_REQUIREMENT(__is_sorted(__key_cont), "Key container is not sorted");
     auto __guard         = std::__make_exception_guard([&]() noexcept { clear() /* noexcept */; });
     __containers_.keys   = std::move(__key_cont);
     __containers_.values = std::move(__mapped_cont);
     __guard.__complete();
   }
 
   _LIBCPP_HIDE_FROM_ABI iterator erase(iterator __position) {
     return __erase(__position.__key_iter_, __position.__mapped_iter_);
   }
 
   _LIBCPP_HIDE_FROM_ABI iterator erase(const_iterator __position) {
     return __erase(__position.__key_iter_, __position.__mapped_iter_);
   }
 
   _LIBCPP_HIDE_FROM_ABI size_type erase(const key_type& __x) {
     auto [__first, __last] = equal_range(__x);
     auto __res             = __last - __first;
     erase(__first, __last);
     return __res;
   }
 
   template <class _Kp>
     requires(__is_compare_transparent && !is_convertible_v<_Kp &&, iterator> &&
              !is_convertible_v<_Kp &&, const_iterator>)
   _LIBCPP_HIDE_FROM_ABI size_type erase(_Kp&& __x) {
     auto [__first, __last] = equal_range(__x);
     auto __res             = __last - __first;
     erase(__first, __last);
     return __res;
   }
 
   _LIBCPP_HIDE_FROM_ABI iterator erase(const_iterator __first, const_iterator __last) {
     auto __on_failure = std::__make_exception_guard([&]() noexcept { clear() /* noexcept */; });
     auto __key_it     = __containers_.keys.erase(__first.__key_iter_, __last.__key_iter_);
     auto __mapped_it  = __containers_.values.erase(__first.__mapped_iter_, __last.__mapped_iter_);
     __on_failure.__complete();
     return iterator(std::move(__key_it), std::move(__mapped_it));
   }
 
   _LIBCPP_HIDE_FROM_ABI void swap(flat_multimap& __y) noexcept {
     // warning: The spec has unconditional noexcept, which means that
     // if any of the following functions throw an exception,
     // std::terminate will be called
     ranges::swap(__compare_, __y.__compare_);
     ranges::swap(__containers_.keys, __y.__containers_.keys);
     ranges::swap(__containers_.values, __y.__containers_.values);
   }
 
   _LIBCPP_HIDE_FROM_ABI void clear() noexcept {
     __containers_.keys.clear();
     __containers_.values.clear();
   }
 
   // observers
   _LIBCPP_HIDE_FROM_ABI key_compare key_comp() const { return __compare_; }
   _LIBCPP_HIDE_FROM_ABI value_compare value_comp() const { return value_compare(__compare_); }
 
   _LIBCPP_HIDE_FROM_ABI const key_container_type& keys() const noexcept { return __containers_.keys; }
   _LIBCPP_HIDE_FROM_ABI const mapped_container_type& values() const noexcept { return __containers_.values; }
 
   // map operations
   _LIBCPP_HIDE_FROM_ABI iterator find(const key_type& __x) { return __find_impl(*this, __x); }
 
   _LIBCPP_HIDE_FROM_ABI const_iterator find(const key_type& __x) const { return __find_impl(*this, __x); }
 
   template <class _Kp>
     requires __is_compare_transparent
   _LIBCPP_HIDE_FROM_ABI iterator find(const _Kp& __x) {
     return __find_impl(*this, __x);
   }
 
   template <class _Kp>
     requires __is_compare_transparent
   _LIBCPP_HIDE_FROM_ABI const_iterator find(const _Kp& __x) const {
     return __find_impl(*this, __x);
   }
 
   _LIBCPP_HIDE_FROM_ABI size_type count(const key_type& __x) const {
     auto [__first, __last] = equal_range(__x);
     return __last - __first;
   }
 
   template <class _Kp>
     requires __is_compare_transparent
   _LIBCPP_HIDE_FROM_ABI size_type count(const _Kp& __x) const {
     auto [__first, __last] = equal_range(__x);
     return __last - __first;
   }
 
   _LIBCPP_HIDE_FROM_ABI bool contains(const key_type& __x) const { return find(__x) != end(); }
 
   template <class _Kp>
     requires __is_compare_transparent
   _LIBCPP_HIDE_FROM_ABI bool contains(const _Kp& __x) const {
     return find(__x) != end();
   }
 
   _LIBCPP_HIDE_FROM_ABI iterator lower_bound(const key_type& __x) { return __lower_bound<iterator>(*this, __x); }
 
   _LIBCPP_HIDE_FROM_ABI const_iterator lower_bound(const key_type& __x) const {
     return __lower_bound<const_iterator>(*this, __x);
   }
 
   template <class _Kp>
     requires __is_compare_transparent
   _LIBCPP_HIDE_FROM_ABI iterator lower_bound(const _Kp& __x) {
     return __lower_bound<iterator>(*this, __x);
   }
 
   template <class _Kp>
     requires __is_compare_transparent
   _LIBCPP_HIDE_FROM_ABI const_iterator lower_bound(const _Kp& __x) const {
     return __lower_bound<const_iterator>(*this, __x);
   }
 
   _LIBCPP_HIDE_FROM_ABI iterator upper_bound(const key_type& __x) { return __upper_bound<iterator>(*this, __x); }
 
   _LIBCPP_HIDE_FROM_ABI const_iterator upper_bound(const key_type& __x) const {
     return __upper_bound<const_iterator>(*this, __x);
   }
 
   template <class _Kp>
     requires __is_compare_transparent
   _LIBCPP_HIDE_FROM_ABI iterator upper_bound(const _Kp& __x) {
     return __upper_bound<iterator>(*this, __x);
   }
 
   template <class _Kp>
     requires __is_compare_transparent
   _LIBCPP_HIDE_FROM_ABI const_iterator upper_bound(const _Kp& __x) const {
     return __upper_bound<const_iterator>(*this, __x);
   }
 
   _LIBCPP_HIDE_FROM_ABI pair<iterator, iterator> equal_range(const key_type& __x) {
     return __equal_range_impl(*this, __x);
   }
 
   _LIBCPP_HIDE_FROM_ABI pair<const_iterator, const_iterator> equal_range(const key_type& __x) const {
     return __equal_range_impl(*this, __x);
   }
 
   template <class _Kp>
     requires __is_compare_transparent
   _LIBCPP_HIDE_FROM_ABI pair<iterator, iterator> equal_range(const _Kp& __x) {
     return __equal_range_impl(*this, __x);
   }
   template <class _Kp>
     requires __is_compare_transparent
   _LIBCPP_HIDE_FROM_ABI pair<const_iterator, const_iterator> equal_range(const _Kp& __x) const {
     return __equal_range_impl(*this, __x);
   }
 
   friend _LIBCPP_HIDE_FROM_ABI bool operator==(const flat_multimap& __x, const flat_multimap& __y) {
     return ranges::equal(__x, __y);
   }
 
   friend _LIBCPP_HIDE_FROM_ABI auto operator<=>(const flat_multimap& __x, const flat_multimap& __y) {
     return std::lexicographical_compare_three_way(
         __x.begin(), __x.end(), __y.begin(), __y.end(), std::__synth_three_way);
   }
 
   friend _LIBCPP_HIDE_FROM_ABI void swap(flat_multimap& __x, flat_multimap& __y) noexcept { __x.swap(__y); }
 
 private:
   struct __ctor_uses_allocator_tag {
     explicit _LIBCPP_HIDE_FROM_ABI __ctor_uses_allocator_tag() = default;
   };
   struct __ctor_uses_allocator_empty_tag {
     explicit _LIBCPP_HIDE_FROM_ABI __ctor_uses_allocator_empty_tag() = default;
   };
 
   template <class _Allocator, class _KeyCont, class _MappedCont, class... _CompArg>
     requires __allocator_ctor_constraint<_Allocator>
   _LIBCPP_HIDE_FROM_ABI
   flat_multimap(__ctor_uses_allocator_tag,
                 const _Allocator& __alloc,
                 _KeyCont&& __key_cont,
                 _MappedCont&& __mapped_cont,
                 _CompArg&&... __comp)
       : __containers_{.keys = std::make_obj_using_allocator<key_container_type>(
                           __alloc, std::forward<_KeyCont>(__key_cont)),
                       .values = std::make_obj_using_allocator<mapped_container_type>(
                           __alloc, std::forward<_MappedCont>(__mapped_cont))},
         __compare_(std::forward<_CompArg>(__comp)...) {}
 
   template <class _Allocator, class... _CompArg>
     requires __allocator_ctor_constraint<_Allocator>
   _LIBCPP_HIDE_FROM_ABI flat_multimap(__ctor_uses_allocator_empty_tag, const _Allocator& __alloc, _CompArg&&... __comp)
       : __containers_{.keys   = std::make_obj_using_allocator<key_container_type>(__alloc),
                       .values = std::make_obj_using_allocator<mapped_container_type>(__alloc)},
         __compare_(std::forward<_CompArg>(__comp)...) {}
 
   _LIBCPP_HIDE_FROM_ABI bool __is_sorted(auto&& __key_container) const {
     return ranges::is_sorted(__key_container, __compare_);
   }
 
   _LIBCPP_HIDE_FROM_ABI void __sort() {
     auto __zv = ranges::views::zip(__containers_.keys, __containers_.values);
     ranges::sort(__zv, __compare_, [](const auto& __p) -> decltype(auto) { return std::get<0>(__p); });
   }
 
   template <class _Self, class _KeyIter>
   _LIBCPP_HIDE_FROM_ABI static auto __corresponding_mapped_it(_Self&& __self, _KeyIter&& __key_iter) {
     return __self.__containers_.values.begin() +
            static_cast<ranges::range_difference_t<mapped_container_type>>(
                ranges::distance(__self.__containers_.keys.begin(), __key_iter));
   }
 
   template <bool _WasSorted, class _InputIterator, class _Sentinel>
   _LIBCPP_HIDE_FROM_ABI void __append_sort_merge(_InputIterator __first, _Sentinel __last) {
     auto __on_failure     = std::__make_exception_guard([&]() noexcept { clear() /* noexcept */; });
     size_t __num_appended = __flat_map_utils::__append(*this, std::move(__first), std::move(__last));
     if (__num_appended != 0) {
       auto __zv                  = ranges::views::zip(__containers_.keys, __containers_.values);
       auto __append_start_offset = __containers_.keys.size() - __num_appended;
       auto __end                 = __zv.end();
       auto __compare_key         = [this](const auto& __p1, const auto& __p2) {
         return __compare_(std::get<0>(__p1), std::get<0>(__p2));
       };
       if constexpr (!_WasSorted) {
         ranges::sort(__zv.begin() + __append_start_offset, __end, __compare_key);
       } else {
         _LIBCPP_ASSERT_SEMANTIC_REQUIREMENT(
             __is_sorted(__containers_.keys | ranges::views::drop(__append_start_offset)),
             "Key container is not sorted");
       }
       ranges::inplace_merge(__zv.begin(), __zv.begin() + __append_start_offset, __end, __compare_key);
     }
     __on_failure.__complete();
   }
 
   template <class _Self, class _Kp>
   _LIBCPP_HIDE_FROM_ABI static auto __find_impl(_Self&& __self, const _Kp& __key) {
     auto __it   = __self.lower_bound(__key);
     auto __last = __self.end();
     if (__it == __last || __self.__compare_(__key, __it->first)) {
       return __last;
     }
     return __it;
   }
 
   template <class _Self, class _Kp>
   _LIBCPP_HIDE_FROM_ABI static auto __equal_range_impl(_Self&& __self, const _Kp& __key) {
     auto [__key_first, __key_last] = ranges::equal_range(__self.__containers_.keys, __key, __self.__compare_);
 
     using __iterator_type = ranges::iterator_t<decltype(__self)>;
     return std::make_pair(__iterator_type(__key_first, __corresponding_mapped_it(__self, __key_first)),
                           __iterator_type(__key_last, __corresponding_mapped_it(__self, __key_last)));
   }
 
   template <class _Res, class _Self, class _Kp>
   _LIBCPP_HIDE_FROM_ABI static _Res __lower_bound(_Self&& __self, _Kp& __x) {
     auto __key_iter    = ranges::lower_bound(__self.__containers_.keys, __x, __self.__compare_);
     auto __mapped_iter = __corresponding_mapped_it(__self, __key_iter);
     return _Res(std::move(__key_iter), std::move(__mapped_iter));
   }
 
   template <class _Res, class _Self, class _Kp>
   _LIBCPP_HIDE_FROM_ABI static _Res __upper_bound(_Self&& __self, _Kp& __x) {
     auto __key_iter    = ranges::upper_bound(__self.__containers_.keys, __x, __self.__compare_);
     auto __mapped_iter = __corresponding_mapped_it(__self, __key_iter);
     return _Res(std::move(__key_iter), std::move(__mapped_iter));
   }
 
   _LIBCPP_HIDE_FROM_ABI void __reserve(size_t __size) {
     if constexpr (requires { __containers_.keys.reserve(__size); }) {
       __containers_.keys.reserve(__size);
     }
 
     if constexpr (requires { __containers_.values.reserve(__size); }) {
       __containers_.values.reserve(__size);
     }
   }
 
   template <class _KIter, class _MIter>
   _LIBCPP_HIDE_FROM_ABI iterator __erase(_KIter __key_iter_to_remove, _MIter __mapped_iter_to_remove) {
     auto __on_failure  = std::__make_exception_guard([&]() noexcept { clear() /* noexcept */; });
     auto __key_iter    = __containers_.keys.erase(__key_iter_to_remove);
     auto __mapped_iter = __containers_.values.erase(__mapped_iter_to_remove);
     __on_failure.__complete();
     return iterator(std::move(__key_iter), std::move(__mapped_iter));
   }
 
   template <class _Key2, class _Tp2, class _Compare2, class _KeyContainer2, class _MappedContainer2, class _Predicate>
   friend typename flat_multimap<_Key2, _Tp2, _Compare2, _KeyContainer2, _MappedContainer2>::size_type
   erase_if(flat_multimap<_Key2, _Tp2, _Compare2, _KeyContainer2, _MappedContainer2>&, _Predicate);
 
   friend __flat_map_utils;
 
   containers __containers_;
   _LIBCPP_NO_UNIQUE_ADDRESS key_compare __compare_;
 
   struct __key_equiv {
     _LIBCPP_HIDE_FROM_ABI __key_equiv(key_compare __c) : __comp_(__c) {}
     _LIBCPP_HIDE_FROM_ABI bool operator()(const_reference __x, const_reference __y) const {
       return !__comp_(std::get<0>(__x), std::get<0>(__y)) && !__comp_(std::get<0>(__y), std::get<0>(__x));
     }
     key_compare __comp_;
   };
 };
 
 template <class _KeyContainer, class _MappedContainer, class _Compare = less<typename _KeyContainer::value_type>>
   requires(!__is_allocator<_Compare>::value && !__is_allocator<_KeyContainer>::value &&
            !__is_allocator<_MappedContainer>::value &&
            is_invocable_v<const _Compare&,
                           const typename _KeyContainer::value_type&,
                           const typename _KeyContainer::value_type&>)
 flat_multimap(_KeyContainer, _MappedContainer, _Compare = _Compare())
     -> flat_multimap<typename _KeyContainer::value_type,
                      typename _MappedContainer::value_type,
                      _Compare,
                      _KeyContainer,
                      _MappedContainer>;
 
 template <class _KeyContainer, class _MappedContainer, class _Allocator>
   requires(uses_allocator_v<_KeyContainer, _Allocator> && uses_allocator_v<_MappedContainer, _Allocator> &&
            !__is_allocator<_KeyContainer>::value && !__is_allocator<_MappedContainer>::value)
 flat_multimap(_KeyContainer, _MappedContainer, _Allocator)
     -> flat_multimap<typename _KeyContainer::value_type,
                      typename _MappedContainer::value_type,
                      less<typename _KeyContainer::value_type>,
                      _KeyContainer,
                      _MappedContainer>;
 
 template <class _KeyContainer, class _MappedContainer, class _Compare, class _Allocator>
   requires(!__is_allocator<_Compare>::value && !__is_allocator<_KeyContainer>::value &&
            !__is_allocator<_MappedContainer>::value && uses_allocator_v<_KeyContainer, _Allocator> &&
            uses_allocator_v<_MappedContainer, _Allocator> &&
            is_invocable_v<const _Compare&,
                           const typename _KeyContainer::value_type&,
                           const typename _KeyContainer::value_type&>)
 flat_multimap(_KeyContainer, _MappedContainer, _Compare, _Allocator)
     -> flat_multimap<typename _KeyContainer::value_type,
                      typename _MappedContainer::value_type,
                      _Compare,
                      _KeyContainer,
                      _MappedContainer>;
 
 template <class _KeyContainer, class _MappedContainer, class _Compare = less<typename _KeyContainer::value_type>>
   requires(!__is_allocator<_Compare>::value && !__is_allocator<_KeyContainer>::value &&
            !__is_allocator<_MappedContainer>::value &&
            is_invocable_v<const _Compare&,
                           const typename _KeyContainer::value_type&,
                           const typename _KeyContainer::value_type&>)
 flat_multimap(sorted_equivalent_t, _KeyContainer, _MappedContainer, _Compare = _Compare())
     -> flat_multimap<typename _KeyContainer::value_type,
                      typename _MappedContainer::value_type,
                      _Compare,
                      _KeyContainer,
                      _MappedContainer>;
 
 template <class _KeyContainer, class _MappedContainer, class _Allocator>
   requires(uses_allocator_v<_KeyContainer, _Allocator> && uses_allocator_v<_MappedContainer, _Allocator> &&
            !__is_allocator<_KeyContainer>::value && !__is_allocator<_MappedContainer>::value)
 flat_multimap(sorted_equivalent_t, _KeyContainer, _MappedContainer, _Allocator)
     -> flat_multimap<typename _KeyContainer::value_type,
                      typename _MappedContainer::value_type,
                      less<typename _KeyContainer::value_type>,
                      _KeyContainer,
                      _MappedContainer>;
 
 template <class _KeyContainer, class _MappedContainer, class _Compare, class _Allocator>
   requires(!__is_allocator<_Compare>::value && !__is_allocator<_KeyContainer>::value &&
            !__is_allocator<_MappedContainer>::value && uses_allocator_v<_KeyContainer, _Allocator> &&
            uses_allocator_v<_MappedContainer, _Allocator> &&
            is_invocable_v<const _Compare&,
                           const typename _KeyContainer::value_type&,
                           const typename _KeyContainer::value_type&>)
 flat_multimap(sorted_equivalent_t, _KeyContainer, _MappedContainer, _Compare, _Allocator)
     -> flat_multimap<typename _KeyContainer::value_type,
                      typename _MappedContainer::value_type,
                      _Compare,
                      _KeyContainer,
                      _MappedContainer>;
 
 template <class _InputIterator, class _Compare = less<__iter_key_type<_InputIterator>>>
   requires(__has_input_iterator_category<_InputIterator>::value && !__is_allocator<_Compare>::value)
 flat_multimap(_InputIterator, _InputIterator, _Compare = _Compare())
     -> flat_multimap<__iter_key_type<_InputIterator>, __iter_mapped_type<_InputIterator>, _Compare>;
 
 template <class _InputIterator, class _Compare = less<__iter_key_type<_InputIterator>>>
   requires(__has_input_iterator_category<_InputIterator>::value && !__is_allocator<_Compare>::value)
 flat_multimap(sorted_equivalent_t, _InputIterator, _InputIterator, _Compare = _Compare())
     -> flat_multimap<__iter_key_type<_InputIterator>, __iter_mapped_type<_InputIterator>, _Compare>;
 
 template <ranges::input_range _Range,
           class _Compare   = less<__range_key_type<_Range>>,
           class _Allocator = allocator<byte>,
           class            = __enable_if_t<!__is_allocator<_Compare>::value && __is_allocator<_Allocator>::value>>
 flat_multimap(from_range_t, _Range&&, _Compare = _Compare(), _Allocator = _Allocator()) -> flat_multimap<
     __range_key_type<_Range>,
     __range_mapped_type<_Range>,
     _Compare,
     vector<__range_key_type<_Range>, __allocator_traits_rebind_t<_Allocator, __range_key_type<_Range>>>,
     vector<__range_mapped_type<_Range>, __allocator_traits_rebind_t<_Allocator, __range_mapped_type<_Range>>>>;
 
 template <ranges::input_range _Range, class _Allocator, class = __enable_if_t<__is_allocator<_Allocator>::value>>
 flat_multimap(from_range_t, _Range&&, _Allocator) -> flat_multimap<
     __range_key_type<_Range>,
     __range_mapped_type<_Range>,
     less<__range_key_type<_Range>>,
     vector<__range_key_type<_Range>, __allocator_traits_rebind_t<_Allocator, __range_key_type<_Range>>>,
     vector<__range_mapped_type<_Range>, __allocator_traits_rebind_t<_Allocator, __range_mapped_type<_Range>>>>;
 
 template <class _Key, class _Tp, class _Compare = less<_Key>>
   requires(!__is_allocator<_Compare>::value)
 flat_multimap(initializer_list<pair<_Key, _Tp>>, _Compare = _Compare()) -> flat_multimap<_Key, _Tp, _Compare>;
 
 template <class _Key, class _Tp, class _Compare = less<_Key>>
   requires(!__is_allocator<_Compare>::value)
 flat_multimap(sorted_equivalent_t, initializer_list<pair<_Key, _Tp>>, _Compare = _Compare())
     -> flat_multimap<_Key, _Tp, _Compare>;
 
 template <class _Key, class _Tp, class _Compare, class _KeyContainer, class _MappedContainer, class _Allocator>
 struct uses_allocator<flat_multimap<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>, _Allocator>
     : bool_constant<uses_allocator_v<_KeyContainer, _Allocator> && uses_allocator_v<_MappedContainer, _Allocator>> {};
 
 template <class _Key, class _Tp, class _Compare, class _KeyContainer, class _MappedContainer, class _Predicate>
 _LIBCPP_HIDE_FROM_ABI typename flat_multimap<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>::size_type
 erase_if(flat_multimap<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>& __flat_multimap, _Predicate __pred) {
   auto __zv     = ranges::views::zip(__flat_multimap.__containers_.keys, __flat_multimap.__containers_.values);
   auto __first  = __zv.begin();
   auto __last   = __zv.end();
   auto __guard  = std::__make_exception_guard([&] { __flat_multimap.clear(); });
   auto __it     = std::remove_if(__first, __last, [&](auto&& __zipped) -> bool {
     using _Ref = typename flat_multimap<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>::const_reference;
     return __pred(_Ref(std::get<0>(__zipped), std::get<1>(__zipped)));
   });
   auto __res    = __last - __it;
   auto __offset = __it - __first;
 
   const auto __erase_container = [&](auto& __cont) { __cont.erase(__cont.begin() + __offset, __cont.end()); };
 
   __erase_container(__flat_multimap.__containers_.keys);
   __erase_container(__flat_multimap.__containers_.values);
 
   __guard.__complete();
   return __res;
 }
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP_STD_VER >= 23
 
 _LIBCPP_POP_MACROS
 
 #endif // _LIBCPP___FLAT_MAP_FLAT_MULTIMAP_H

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