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diff --git a/third_party/immer/immer/detail/rbts/visitor.hpp b/third_party/immer/immer/detail/rbts/visitor.hpp
new file mode 100644
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+++ b/third_party/immer/immer/detail/rbts/visitor.hpp
@@ -0,0 +1,56 @@
+//
+// 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/config.hpp>
+
+#include <tuple>
+#include <utility>
+
+namespace immer {
+namespace detail {
+namespace rbts {
+
+template <typename Deriv>
+struct visitor_base
+{
+    template <typename... Args>
+    static decltype(auto) visit_node(Args&&... args)
+    {
+        IMMER_UNREACHABLE;
+    }
+
+    template <typename... Args>
+    static decltype(auto) visit_relaxed(Args&&... args)
+    {
+        return Deriv::visit_inner(std::forward<Args>(args)...);
+    }
+
+    template <typename... Args>
+    static decltype(auto) visit_regular(Args&&... args)
+    {
+        return Deriv::visit_inner(std::forward<Args>(args)...);
+    }
+
+    template <typename... Args>
+    static decltype(auto) visit_inner(Args&&... args)
+    {
+        return Deriv::visit_node(std::forward<Args>(args)...);
+    }
+
+    template <typename... Args>
+    static decltype(auto) visit_leaf(Args&&... args)
+    {
+        return Deriv::visit_node(std::forward<Args>(args)...);
+    }
+};
+
+} // namespace rbts
+} // namespace detail
+} // namespace immer