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diff --git a/third_party/immer/example/set/intro.cpp b/third_party/immer/example/set/intro.cpp
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+++ b/third_party/immer/example/set/intro.cpp
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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
+//
+
+// include:intro/start
+#include <immer/set.hpp>
+int main()
+{
+    const auto v0 = immer::set<int>{};
+    const auto v1 = v0.insert(42);
+    assert(v0.count(42) == 0);
+    assert(v1.count(42) == 1);
+
+    const auto v2 = v1.erase(42);
+    assert(v1.count(42) == 1);
+    assert(v2.count(42) == 0);
+}
+// include:intro/end