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diff --git a/third_party/immer/immer/heap/split_heap.hpp b/third_party/immer/immer/heap/split_heap.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 <atomic>
+#include <cassert>
+
+namespace immer {
+
+/*!
+ * Adaptor that uses `SmallHeap` for allocations that are smaller or
+ * equal to `Size` and `BigHeap` otherwise.
+ */
+template <std::size_t Size, typename SmallHeap, typename BigHeap>
+struct split_heap
+{
+    template <typename... Tags>
+    static void* allocate(std::size_t size, Tags... tags)
+    {
+        return size <= Size ? SmallHeap::allocate(size, tags...)
+                            : BigHeap::allocate(size, tags...);
+    }
+
+    template <typename... Tags>
+    static void deallocate(std::size_t size, void* data, Tags... tags)
+    {
+        if (size <= Size)
+            SmallHeap::deallocate(size, data, tags...);
+        else
+            BigHeap::deallocate(size, data, tags...);
+    }
+};
+
+} // namespace immer