Squashed 'third_party/immer/' content from commit ad3e3556d

git-subtree-dir: third_party/immer
git-subtree-split: ad3e3556d38bb75966dd24c61a774970a7c7957e

example/map/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 <string>
+// include:intro/start
+#include <immer/map.hpp>
+int main()
+{
+    const auto v0 = immer::map<std::string, int>{};
+    const auto v1 = v0.set("hello", 42);
+    assert(v0["hello"] == 0);
+    assert(v1["hello"] == 42);
+
+    const auto v2 = v1.erase("hello");
+    assert(*v1.find("hello") == 42);
+    assert(!v2.find("hello"));
+}
+// include:intro/end