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diff --git a/third_party/immer/immer/refcount/unsafe_refcount_policy.hpp b/third_party/immer/immer/refcount/unsafe_refcount_policy.hpp
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+++ b/third_party/immer/immer/refcount/unsafe_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 <utility>
+
+namespace immer {
+
+/*!
+ * A reference counting policy implemented using a raw `int` count.
+ * It is **not thread-safe**.
+ */
+struct unsafe_refcount_policy
+{
+    using spinlock_type = no_spinlock;
+
+    mutable int refcount;
+
+    unsafe_refcount_policy()
+        : refcount{1} {};
+    unsafe_refcount_policy(disowned)
+        : refcount{0}
+    {}
+
+    void inc() { ++refcount; }
+    bool dec() { return --refcount == 0; }
+    void dec_unsafe() { --refcount; }
+    bool unique() { return refcount == 1; }
+};
+
+} // namespace immer