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diff --git a/src/boost/libs/yap/example/let.cpp b/src/boost/libs/yap/example/let.cpp
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+++ b/src/boost/libs/yap/example/let.cpp
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+// Copyright (C) 2016-2018 T. Zachary Laine
+//
+// Distributed under the Boost Software License, Version 1.0. (See
+// accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+//[ let
+#include <boost/yap/yap.hpp>
+
+#include <boost/hana/map.hpp>
+#include <boost/hana/at_key.hpp>
+#include <boost/hana/contains.hpp>
+#include <boost/hana/keys.hpp>
+
+#include <vector>
+#include <iostream>
+
+
+// Here, we introduce special let-placeholders, so we can use them along side
+// the normal YAP placeholders without getting them confused.
+template<long long I>
+struct let_placeholder : boost::hana::llong<I>
+{
+};
+
+// Replaces each let-terminal with the expression with which it was
+// initialized in let().  So in 'let(_a = foo)[ _a + 1 ]', this transform will
+// be used on '_a + 1' to replace '_a' with 'foo'.  The map_ member holds the
+// mapping of let-placeholders to their initializers.
+template<typename ExprMap>
+struct let_terminal_transform
+{
+    // This matches only let-placeholders.  For each one matched, we look up
+    // its initializer in map_ and return it.
+    template<long long I>
+    auto operator()(
+        boost::yap::expr_tag<boost::yap::expr_kind::terminal>,
+        let_placeholder<I> i)
+    {
+        // If we have an entry in map_ for this placeholder, return the value
+        // of the entry.  Otherwise, pass i through as a terminal.
+        if constexpr (boost::hana::contains(
+                          decltype(boost::hana::keys(map_))(),
+                          boost::hana::llong_c<I>)) {
+            return map_[boost::hana::llong_c<I>];
+        } else {
+            return boost::yap::make_terminal(i);
+        }
+    }
+
+    ExprMap map_;
+};
+
+// As you can see below, let() is an eager function; this template is used for
+// its return values.  It contains the mapping from let-placeholders to
+// initializer expressions used to transform the expression inside '[]' after
+// a let()'.  It also has an operator[]() member function that takes the
+// expression inside '[]' and returns a version of it with the
+// let-placeholders replaced.
+template<typename ExprMap>
+struct let_result
+{
+    template<typename Expr>
+    auto operator[](Expr && expr)
+    {
+        return boost::yap::transform(
+            std::forward<Expr>(expr), let_terminal_transform<ExprMap>{map_});
+    }
+
+    ExprMap map_;
+};
+
+// Processes the expressions passed to let() one at a time, adding each one to
+// a Hana map of hana::llong<>s to YAP expressions.
+template<typename Map, typename Expr, typename... Exprs>
+auto let_impl(Map && map, Expr && expr, Exprs &&... exprs)
+{
+    static_assert(
+        Expr::kind == boost::yap::expr_kind::assign,
+        "Expressions passed to let() must be of the form placeholder = Expression");
+    if constexpr (sizeof...(Exprs) == 0) {
+        using I = typename std::remove_reference<decltype(
+            boost::yap::value(boost::yap::left(expr)))>::type;
+        auto const i = boost::hana::llong_c<I::value>;
+        using map_t = decltype(boost::hana::insert(
+            map, boost::hana::make_pair(i, boost::yap::right(expr))));
+        return let_result<map_t>{boost::hana::insert(
+            map, boost::hana::make_pair(i, boost::yap::right(expr)))};
+    } else {
+        using I = typename std::remove_reference<decltype(
+            boost::yap::value(boost::yap::left(expr)))>::type;
+        auto const i = boost::hana::llong_c<I::value>;
+        return let_impl(
+            boost::hana::insert(
+                map, boost::hana::make_pair(i, boost::yap::right(expr))),
+            std::forward<Exprs>(exprs)...);
+    }
+}
+
+// Takes N > 0 expressions of the form 'placeholder = expr', and returns an
+// object with an overloaded operator[]().
+template<typename Expr, typename... Exprs>
+auto let(Expr && expr, Exprs &&... exprs)
+{
+    return let_impl(
+        boost::hana::make_map(),
+        std::forward<Expr>(expr),
+        std::forward<Exprs>(exprs)...);
+}
+
+int main()
+{
+    // Some handy terminals -- the _a and _b let-placeholders and std::cout as
+    // a YAP terminal.
+    boost::yap::expression<
+        boost::yap::expr_kind::terminal,
+        boost::hana::tuple<let_placeholder<0>>> const _a;
+    boost::yap::expression<
+        boost::yap::expr_kind::terminal,
+        boost::hana::tuple<let_placeholder<1>>> const _b;
+    auto const cout = boost::yap::make_terminal(std::cout);
+
+    using namespace boost::yap::literals;
+
+    {
+        auto expr = let(_a = 2)[_a + 1];
+        assert(boost::yap::evaluate(expr) == 3);
+    }
+
+    {
+        auto expr = let(_a = 123, _b = 456)[_a + _b];
+        assert(boost::yap::evaluate(expr) == 123 + 456);
+    }
+
+    // This prints out "0 0", because 'i' is passed as an lvalue, so its
+    // decrement is visible outside the let expression.
+    {
+        int i = 1;
+
+        boost::yap::evaluate(let(_a = 1_p)[cout << --_a << ' '], i);
+
+        std::cout << i << std::endl;
+    }
+
+    // Prints "Hello, World" due to let()'s scoping rules.
+    {
+        boost::yap::evaluate(
+            let(_a = 1_p, _b = 2_p)
+            [
+                // _a here is an int: 1
+
+                let(_a = 3_p) // hides the outer _a
+                [
+                    cout << _a << _b // prints "Hello, World"
+                ]
+            ],
+            1, " World", "Hello,"
+        );
+    }
+
+    std::cout << "\n";
+
+    // Due to the macro-substitution style that this example uses, this prints
+    // "3132".  Phoenix's let() prints "312", because it only evaluates '1_p
+    // << 3' once.
+    {
+        boost::yap::evaluate(
+            let(_a = 1_p << 3)
+            [
+                _a << "1", _a << "2"
+            ],
+            std::cout
+        );
+    }
+
+    std::cout << "\n";
+}
+//]