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diff --git a/src/boost/libs/yap/example/tarray.cpp b/src/boost/libs/yap/example/tarray.cpp
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+++ b/src/boost/libs/yap/example/tarray.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)
+//[ tarray
+#include <boost/yap/algorithm.hpp>
+#include <boost/yap/print.hpp>
+
+#include <array>
+#include <iostream>
+
+
+template <boost::yap::expr_kind Kind, typename Tuple>
+struct tarray_expr;
+
+
+struct take_nth
+{
+    boost::yap::terminal<tarray_expr, int>
+    operator() (boost::yap::terminal<tarray_expr, std::array<int, 3>> const & expr);
+
+    std::size_t n;
+};
+
+// Another custom expression template.  In this case, we static_assert() that
+// it only gets instantiated with terminals with pre-approved value types.
+template <boost::yap::expr_kind Kind, typename Tuple>
+struct tarray_expr
+{
+    // Make sure that, if this expression is a terminal, its value is one we
+    // want to support.  Note that the presence of expr_kind::expr_ref makes
+    // life slightly more difficult; we have to account for int const & and
+    // int & as well as int.
+    static_assert(
+        Kind != boost::yap::expr_kind::terminal ||
+        std::is_same<Tuple, boost::hana::tuple<int const &>>{} ||
+        std::is_same<Tuple, boost::hana::tuple<int &>>{} ||
+        std::is_same<Tuple, boost::hana::tuple<int>>{} ||
+        std::is_same<Tuple, boost::hana::tuple<std::array<int, 3>>>{},
+        "tarray_expr instantiated with an unsupported terminal type."
+    );
+
+    static const boost::yap::expr_kind kind = Kind;
+
+    Tuple elements;
+
+    int operator[] (std::size_t n) const
+    { return boost::yap::evaluate(boost::yap::transform(*this, take_nth{n})); }
+};
+
+// Define operators +, -, *, and /.
+BOOST_YAP_USER_BINARY_OPERATOR(plus, tarray_expr, tarray_expr)
+BOOST_YAP_USER_BINARY_OPERATOR(minus, tarray_expr, tarray_expr)
+BOOST_YAP_USER_BINARY_OPERATOR(multiplies, tarray_expr, tarray_expr)
+BOOST_YAP_USER_BINARY_OPERATOR(divides, tarray_expr, tarray_expr)
+
+
+boost::yap::terminal<tarray_expr, int>
+take_nth::operator() (boost::yap::terminal<tarray_expr, std::array<int, 3>> const & expr)
+{
+    int x = boost::yap::value(expr)[n];
+    // Again, this is the move hack to get x into the resulting terminal as a
+    // value instead of a reference.
+    return boost::yap::make_terminal<tarray_expr>(std::move(x));
+}
+
+
+// Stream-out operators for the two kinds of terminals we support.
+
+std::ostream & operator<< (std::ostream & os, boost::yap::terminal<tarray_expr, int> expr)
+{ return os << '{' << boost::yap::value(expr) << '}'; }
+
+std::ostream & operator<< (std::ostream & os, boost::yap::terminal<tarray_expr, std::array<int, 3>> expr)
+{
+    std::array<int, 3> const & a = boost::yap::value(expr);
+    return os << '{' << a[0] << ", " << a[1] << ", " << a[2] << '}';
+}
+
+// Stream-out operators for general expressions.  Note that we have to treat
+// the reference case separately; this also could have been done using
+// constexpr if in a single function template.
+
+template <typename Tuple>
+std::ostream & operator<< (std::ostream & os, tarray_expr<boost::yap::expr_kind::expr_ref, Tuple> const & expr)
+{ return os << boost::yap::deref(expr); }
+
+template <boost::yap::expr_kind Kind, typename Tuple>
+std::ostream & operator<< (std::ostream & os, tarray_expr<Kind, Tuple> const & expr)
+{
+    if (Kind == boost::yap::expr_kind::plus || Kind == boost::yap::expr_kind::minus)
+        os << '(';
+    os << boost::yap::left(expr) << " " << op_string(Kind) << " " << boost::yap::right(expr);
+    if (Kind == boost::yap::expr_kind::plus || Kind == boost::yap::expr_kind::minus)
+        os << ')';
+    return os;
+}
+
+
+// Since we want different behavior on terminals than on other kinds of
+// expressions, we create a custom type that does so.
+struct tarray :
+    tarray_expr<
+        boost::yap::expr_kind::terminal,
+        boost::hana::tuple<std::array<int, 3>>
+    >
+{
+    explicit tarray (int i = 0, int j = 0, int k = 0)
+    {
+        (*this)[0] = i;
+        (*this)[1] = j;
+        (*this)[2] = k;
+    }
+
+    explicit tarray (std::array<int, 3> a)
+    {
+        (*this)[0] = a[0];
+        (*this)[1] = a[1];
+        (*this)[2] = a[2];
+    }
+
+    int & operator[] (std::ptrdiff_t i)
+    { return boost::yap::value(*this)[i]; }
+
+    int const & operator[] (std::ptrdiff_t i) const
+    { return boost::yap::value(*this)[i]; }
+
+    template <typename T>
+    tarray & operator= (T const & t)
+    {
+        // We use as_expr() here to make sure that the value passed to
+        // assign() is an expression.  as_expr() simply forwards expressions
+        // through, and wraps non-expressions as terminals.
+        return assign(boost::yap::as_expr< ::tarray_expr>(t));
+    }
+
+    template <typename Expr>
+    tarray & printAssign (Expr const & expr)
+    {
+        *this = expr;
+        std::cout << *this << " = " << expr << std::endl;
+        return *this;
+    }
+
+private:
+    template <typename Expr>
+    tarray & assign (Expr const & expr)
+    {
+        (*this)[0] = expr[0];
+        (*this)[1] = expr[1];
+        (*this)[2] = expr[2];
+        return *this;
+    }
+};
+
+
+int main()
+{
+    tarray a(3,1,2);
+
+    tarray b;
+
+    std::cout << a << std::endl;
+    std::cout << b << std::endl;
+
+    b[0] = 7; b[1] = 33; b[2] = -99;
+
+    tarray c(a);
+
+    std::cout << c << std::endl;
+
+    a = 0;
+
+    std::cout << a << std::endl;
+    std::cout << b << std::endl;
+    std::cout << c << std::endl;
+
+    a = b + c;
+
+    std::cout << a << std::endl;
+
+    a.printAssign(b+c*(b + 3*c));
+
+    return 0;
+}
+//]