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diff --git a/third_party/immer/immer/atom.hpp b/third_party/immer/immer/atom.hpp
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+++ b/third_party/immer/immer/atom.hpp
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+//
+// 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/box.hpp>
+#include <immer/refcount/no_refcount_policy.hpp>
+
+#include <atomic>
+#include <type_traits>
+
+namespace immer {
+
+namespace detail {
+
+template <typename T, typename MemoryPolicy>
+struct refcount_atom_impl
+{
+    using box_type      = box<T, MemoryPolicy>;
+    using value_type    = T;
+    using memory_policy = MemoryPolicy;
+    using spinlock_t    = typename MemoryPolicy::refcount::spinlock_type;
+    using scoped_lock_t = typename spinlock_t::scoped_lock;
+
+    refcount_atom_impl(const refcount_atom_impl&) = delete;
+    refcount_atom_impl(refcount_atom_impl&&)      = delete;
+    refcount_atom_impl& operator=(const refcount_atom_impl&) = delete;
+    refcount_atom_impl& operator=(refcount_atom_impl&&) = delete;
+
+    refcount_atom_impl(box_type b)
+        : impl_{std::move(b)}
+    {}
+
+    box_type load() const
+    {
+        scoped_lock_t lock{lock_};
+        return impl_;
+    }
+
+    void store(box_type b)
+    {
+        scoped_lock_t lock{lock_};
+        impl_ = std::move(b);
+    }
+
+    box_type exchange(box_type b)
+    {
+        {
+            scoped_lock_t lock{lock_};
+            swap(b, impl_);
+        }
+        return b;
+    }
+
+    template <typename Fn>
+    box_type update(Fn&& fn)
+    {
+        while (true) {
+            auto oldv = load();
+            auto newv = oldv.update(fn);
+            {
+                scoped_lock_t lock{lock_};
+                if (oldv.impl_ == impl_.impl_) {
+                    impl_ = newv;
+                    return {newv};
+                }
+            }
+        }
+    }
+
+private:
+    mutable spinlock_t lock_;
+    box_type impl_;
+};
+
+template <typename T, typename MemoryPolicy>
+struct gc_atom_impl
+{
+    using box_type      = box<T, MemoryPolicy>;
+    using value_type    = T;
+    using memory_policy = MemoryPolicy;
+
+    static_assert(std::is_same<typename MemoryPolicy::refcount,
+                               no_refcount_policy>::value,
+                  "gc_atom_impl can only be used when there is no refcount!");
+
+    gc_atom_impl(const gc_atom_impl&) = delete;
+    gc_atom_impl(gc_atom_impl&&)      = delete;
+    gc_atom_impl& operator=(const gc_atom_impl&) = delete;
+    gc_atom_impl& operator=(gc_atom_impl&&) = delete;
+
+    gc_atom_impl(box_type b)
+        : impl_{b.impl_}
+    {}
+
+    box_type load() const { return {impl_.load()}; }
+
+    void store(box_type b) { impl_.store(b.impl_); }
+
+    box_type exchange(box_type b) { return {impl_.exchange(b.impl_)}; }
+
+    template <typename Fn>
+    box_type update(Fn&& fn)
+    {
+        while (true) {
+            auto oldv = box_type{impl_.load()};
+            auto newv = oldv.update(fn);
+            if (impl_.compare_exchange_weak(oldv.impl_, newv.impl_))
+                return {newv};
+        }
+    }
+
+private:
+    std::atomic<typename box_type::holder*> impl_;
+};
+
+} // namespace detail
+
+/*!
+ * Stores for boxed values of type `T` in a thread-safe manner.
+ *
+ * @see box
+ *
+ * @rst
+ *
+ * .. warning:: If memory policy used includes thread unsafe reference counting,
+ *    no no thread safety is assumed, and the atom becomes thread unsafe too!
+ *
+ * .. note:: ``box<T>`` provides a value based box of type ``T``, this is, we
+ * can think about it as a value-based version of ``std::shared_ptr``.  In a
+ *    similar fashion, ``atom<T>`` is in spirit the value-based equivalent of
+ *    C++20 ``std::atomic_shared_ptr``.  However, the API does not follow
+ *    ``std::atomic`` interface closely, since it attempts to be a higher level
+ *    construction, most similar to Clojure's ``(atom)``.  It is remarkable in
+ *    particular that, since ``box<T>`` underlying object is immutable, using
+ *    ``atom<T>`` is fully thread-safe in ways that ``std::atmic_shared_ptr`` is
+ *    not. This is so because dereferencing the underlying pointer in a
+ *    ``std::atomic_share_ptr`` may require further synchronization, in
+ * particular when invoking non-const methods.
+ *
+ * @endrst
+ */
+template <typename T, typename MemoryPolicy = default_memory_policy>
+class atom
+{
+public:
+    using box_type      = box<T, MemoryPolicy>;
+    using value_type    = T;
+    using memory_policy = MemoryPolicy;
+
+    atom(const atom&) = delete;
+    atom(atom&&)      = delete;
+    void operator=(const atom&) = delete;
+    void operator=(atom&&) = delete;
+
+    /*!
+     * Constructs an atom holding a value `b`;
+     */
+    atom(box_type v = {})
+        : impl_{std::move(v)}
+    {}
+
+    /*!
+     * Sets a new value in the atom.
+     */
+    atom& operator=(box_type b)
+    {
+        impl_.store(std::move(b));
+        return *this;
+    }
+
+    /*!
+     * Reads the currently stored value in a thread-safe manner.
+     */
+    operator box_type() const { return impl_.load(); }
+
+    /*!
+     * Reads the currently stored value in a thread-safe manner.
+     */
+    operator value_type() const { return *impl_.load(); }
+
+    /*!
+     * Reads the currently stored value in a thread-safe manner.
+     */
+    IMMER_NODISCARD box_type load() const { return impl_.load(); }
+
+    /*!
+     * Stores a new value in a thread-safe manner.
+     */
+    void store(box_type b) { impl_.store(std::move(b)); }
+
+    /*!
+     * Stores a new value and returns the old value, in a thread-safe manner.
+     */
+    IMMER_NODISCARD box_type exchange(box_type b)
+    {
+        return impl_.exchange(std::move(b));
+    }
+
+    /*!
+     * Stores the result of applying `fn` to the current value atomically and
+     * returns the new resulting value.
+     *
+     * @rst
+     *
+     * .. warning:: ``fn`` must be a pure function and have no side effects! The
+     *    function might be evaluated multiple times when multiple threads
+     *    content to update the value.
+     *
+     * @endrst
+     */
+    template <typename Fn>
+    box_type update(Fn&& fn)
+    {
+        return impl_.update(std::forward<Fn>(fn));
+    }
+
+private:
+    struct get_refcount_atom_impl
+    {
+        template <typename U, typename MP>
+        struct apply
+        {
+            using type = detail::refcount_atom_impl<U, MP>;
+        };
+    };
+
+    struct get_gc_atom_impl
+    {
+        template <typename U, typename MP>
+        struct apply
+        {
+            using type = detail::gc_atom_impl<U, MP>;
+        };
+    };
+
+    // If we are using "real" garbage collection (we assume this when we use
+    // `no_refcount_policy`), we just store the pointer in an atomic.  If we use
+    // reference counting, we rely on the reference counting spinlock.
+    using impl_t = typename std::conditional_t<
+        std::is_same<typename MemoryPolicy::refcount,
+                     no_refcount_policy>::value,
+        get_gc_atom_impl,
+        get_refcount_atom_impl>::template apply<T, MemoryPolicy>::type;
+
+    impl_t impl_;
+};
+
+} // namespace immer