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diff --git a/third_party/immer/immer/refcount/refcount_policy.hpp b/third_party/immer/immer/refcount/refcount_policy.hpp
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+++ b/third_party/immer/immer/refcount/refcount_policy.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/refcount/no_refcount_policy.hpp>
+
+#include <atomic>
+#include <cassert>
+#include <thread>
+#include <utility>
+
+// This has been shamelessly copied from boost...
+#if defined(_MSC_VER) && _MSC_VER >= 1310 &&                                   \
+    (defined(_M_IX86) || defined(_M_X64)) && !defined(__c2__)
+extern "C" void _mm_pause();
+#define IMMER_SMT_PAUSE _mm_pause()
+#elif defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))
+#define IMMER_SMT_PAUSE __asm__ __volatile__("rep; nop" : : : "memory")
+#endif
+
+namespace immer {
+
+// This is an atomic spinlock similar to the one used by boost to provide
+// "atomic" shared_ptr operations.  It also does not differ much from the one
+// from libc++ or libstdc++...
+struct spinlock
+{
+    std::atomic_flag v_{};
+
+    bool try_lock() { return !v_.test_and_set(std::memory_order_acquire); }
+
+    void lock()
+    {
+        for (auto k = 0u; !try_lock(); ++k) {
+            if (k < 4)
+                continue;
+#ifdef IMMER_SMT_PAUSE
+            else if (k < 16)
+                IMMER_SMT_PAUSE;
+#endif
+            else
+                std::this_thread::yield();
+        }
+    }
+
+    void unlock() { v_.clear(std::memory_order_release); }
+
+    struct scoped_lock
+    {
+        scoped_lock(const scoped_lock&) = delete;
+        scoped_lock& operator=(const scoped_lock&) = delete;
+
+        explicit scoped_lock(spinlock& sp)
+            : sp_{sp}
+        {
+            sp.lock();
+        }
+
+        ~scoped_lock() { sp_.unlock(); }
+
+    private:
+        spinlock& sp_;
+    };
+};
+
+/*!
+ * A reference counting policy implemented using an *atomic* `int`
+ * count.  It is **thread-safe**.
+ */
+struct refcount_policy
+{
+    using spinlock_type = spinlock;
+
+    mutable std::atomic<int> refcount;
+
+    refcount_policy()
+        : refcount{1} {};
+    refcount_policy(disowned)
+        : refcount{0}
+    {}
+
+    void inc() { refcount.fetch_add(1, std::memory_order_relaxed); }
+
+    bool dec() { return 1 == refcount.fetch_sub(1, std::memory_order_acq_rel); }
+
+    void dec_unsafe()
+    {
+        assert(refcount.load() > 1);
+        refcount.fetch_sub(1, std::memory_order_relaxed);
+    }
+
+    bool unique() { return refcount == 1; }
+};
+
+} // namespace immer