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diff --git a/third_party/immer/immer/detail/arrays/no_capacity.hpp b/third_party/immer/immer/detail/arrays/no_capacity.hpp
new file mode 100644
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+++ b/third_party/immer/immer/detail/arrays/no_capacity.hpp
@@ -0,0 +1,203 @@
+//
+// immer: immutable data structures for C++
+// Copyright (C) 2016, 2017, 2018 Juan Pedro Bolivar Puente
+//
+// This software is distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE or copy at http://boost.org/LICENSE_1_0.txt
+//
+
+#pragma once
+
+#include <immer/algorithm.hpp>
+#include <immer/detail/arrays/node.hpp>
+
+namespace immer {
+namespace detail {
+namespace arrays {
+
+template <typename T, typename MemoryPolicy>
+struct no_capacity
+{
+    using node_t = node<T, MemoryPolicy>;
+    using edit_t = typename MemoryPolicy::transience_t::edit;
+    using size_t = std::size_t;
+
+    node_t* ptr;
+    size_t size;
+
+    static const no_capacity& empty()
+    {
+        static const no_capacity empty_{
+            node_t::make_n(0),
+            0,
+        };
+        return empty_;
+    }
+
+    no_capacity(node_t* p, size_t s)
+        : ptr{p}
+        , size{s}
+    {}
+
+    no_capacity(const no_capacity& other)
+        : no_capacity{other.ptr, other.size}
+    {
+        inc();
+    }
+
+    no_capacity(no_capacity&& other)
+        : no_capacity{empty()}
+    {
+        swap(*this, other);
+    }
+
+    no_capacity& operator=(const no_capacity& other)
+    {
+        auto next = other;
+        swap(*this, next);
+        return *this;
+    }
+
+    no_capacity& operator=(no_capacity&& other)
+    {
+        swap(*this, other);
+        return *this;
+    }
+
+    friend void swap(no_capacity& x, no_capacity& y)
+    {
+        using std::swap;
+        swap(x.ptr, y.ptr);
+        swap(x.size, y.size);
+    }
+
+    ~no_capacity() { dec(); }
+
+    void inc()
+    {
+        using immer::detail::get;
+        ptr->refs().inc();
+    }
+
+    void dec()
+    {
+        using immer::detail::get;
+        if (ptr->refs().dec())
+            node_t::delete_n(ptr, size, size);
+    }
+
+    T* data() { return ptr->data(); }
+    const T* data() const { return ptr->data(); }
+
+    T* data_mut(edit_t e)
+    {
+        if (!ptr->can_mutate(e))
+            ptr = node_t::copy_e(e, size, ptr, size);
+        return data();
+    }
+
+    template <typename Iter,
+              typename Sent,
+              std::enable_if_t<is_forward_iterator_v<Iter> &&
+                                   compatible_sentinel_v<Iter, Sent>,
+                               bool> = true>
+    static no_capacity from_range(Iter first, Sent last)
+    {
+        auto count = static_cast<size_t>(distance(first, last));
+        if (count == 0)
+            return empty();
+        else
+            return {
+                node_t::copy_n(count, first, last),
+                count,
+            };
+    }
+
+    static no_capacity from_fill(size_t n, T v)
+    {
+        return {node_t::fill_n(n, v), n};
+    }
+
+    template <typename U>
+    static no_capacity from_initializer_list(std::initializer_list<U> values)
+    {
+        using namespace std;
+        return from_range(begin(values), end(values));
+    }
+
+    template <typename Fn>
+    void for_each_chunk(Fn&& fn) const
+    {
+        std::forward<Fn>(fn)(data(), data() + size);
+    }
+
+    template <typename Fn>
+    bool for_each_chunk_p(Fn&& fn) const
+    {
+        return std::forward<Fn>(fn)(data(), data() + size);
+    }
+
+    const T& get(std::size_t index) const { return data()[index]; }
+
+    const T& get_check(std::size_t index) const
+    {
+        if (index >= size)
+            throw std::out_of_range{"out of range"};
+        return data()[index];
+    }
+
+    bool equals(const no_capacity& other) const
+    {
+        return ptr == other.ptr ||
+               (size == other.size &&
+                std::equal(data(), data() + size, other.data()));
+    }
+
+    no_capacity push_back(T value) const
+    {
+        auto p = node_t::copy_n(size + 1, ptr, size);
+        try {
+            new (p->data() + size) T{std::move(value)};
+            return {p, size + 1};
+        } catch (...) {
+            node_t::delete_n(p, size, size + 1);
+            throw;
+        }
+    }
+
+    no_capacity assoc(std::size_t idx, T value) const
+    {
+        auto p = node_t::copy_n(size, ptr, size);
+        try {
+            p->data()[idx] = std::move(value);
+            return {p, size};
+        } catch (...) {
+            node_t::delete_n(p, size, size);
+            throw;
+        }
+    }
+
+    template <typename Fn>
+    no_capacity update(std::size_t idx, Fn&& op) const
+    {
+        auto p = node_t::copy_n(size, ptr, size);
+        try {
+            auto& elem = p->data()[idx];
+            elem       = std::forward<Fn>(op)(std::move(elem));
+            return {p, size};
+        } catch (...) {
+            node_t::delete_n(p, size, size);
+            throw;
+        }
+    }
+
+    no_capacity take(std::size_t sz) const
+    {
+        auto p = node_t::copy_n(sz, ptr, sz);
+        return {p, sz};
+    }
+};
+
+} // namespace arrays
+} // namespace detail
+} // namespace immer