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diff --git a/third_party/immer/immer/heap/tags.hpp b/third_party/immer/immer/heap/tags.hpp
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index 0000000000..d1ce48d05c
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+++ b/third_party/immer/immer/heap/tags.hpp
@@ -0,0 +1,16 @@
+//
+// 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
+
+namespace immer {
+
+struct norefs_tag
+{};
+
+} // namespace immer