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diff --git a/src/boost/libs/yap/example/calc3.cpp b/src/boost/libs/yap/example/calc3.cpp
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+++ b/src/boost/libs/yap/example/calc3.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)
+//[ calc3
+#include <boost/yap/expression.hpp>
+
+#include <boost/hana/maximum.hpp>
+
+#include <iostream>
+
+
+// Look! A transform!  This one transforms the expression tree into the arity
+// of the expression, based on its placeholders.
+//[ calc3_get_arity_xform
+struct get_arity
+{
+    // Base case 1: Match a placeholder terminal, and return its arity as the
+    // result.
+    template <long long I>
+    boost::hana::llong<I> operator() (boost::yap::expr_tag<boost::yap::expr_kind::terminal>,
+                                      boost::yap::placeholder<I>)
+    { return boost::hana::llong_c<I>; }
+
+    // Base case 2: Match any other terminal.  Return 0; non-placeholders do
+    // not contribute to arity.
+    template <typename T>
+    auto operator() (boost::yap::expr_tag<boost::yap::expr_kind::terminal>, T &&)
+    {
+        using namespace boost::hana::literals;
+        return 0_c;
+    }
+
+    // Recursive case: Match any expression not covered above, and return the
+    // maximum of its children's arities.
+    template <boost::yap::expr_kind Kind, typename... Arg>
+    auto operator() (boost::yap::expr_tag<Kind>, Arg &&... arg)
+    {
+        return boost::hana::maximum(
+            boost::hana::make_tuple(
+                boost::yap::transform(
+                    boost::yap::as_expr(std::forward<Arg>(arg)),
+                    get_arity{}
+                )...
+            )
+        );
+    }
+};
+//]
+
+int main ()
+{
+    using namespace boost::yap::literals;
+
+    // These lambdas wrap our expressions as callables, and allow us to check
+    // the arity of each as we call it.
+
+    auto expr_1 = 1_p + 2.0;
+
+    auto expr_1_fn = [expr_1](auto &&... args) {
+        auto const arity = boost::yap::transform(expr_1, get_arity{});
+        static_assert(arity.value == sizeof...(args), "Called with wrong number of args.");
+        return evaluate(expr_1, args...);
+    };
+
+    auto expr_2 = 1_p * 2_p;
+
+    auto expr_2_fn = [expr_2](auto &&... args) {
+        auto const arity = boost::yap::transform(expr_2, get_arity{});
+        static_assert(arity.value == sizeof...(args), "Called with wrong number of args.");
+        return evaluate(expr_2, args...);
+    };
+
+    auto expr_3 = (1_p - 2_p) / 2_p;
+
+    auto expr_3_fn = [expr_3](auto &&... args) {
+        auto const arity = boost::yap::transform(expr_3, get_arity{});
+        static_assert(arity.value == sizeof...(args), "Called with wrong number of args.");
+        return evaluate(expr_3, args...);
+    };
+
+    // Displays "5"
+    std::cout << expr_1_fn(3.0) << std::endl;
+
+    // Displays "6"
+    std::cout << expr_2_fn(3.0, 2.0) << std::endl;
+
+    // Displays "0.5"
+    std::cout << expr_3_fn(3.0, 2.0) << std::endl;
+
+    // Static-asserts with "Called with wrong number of args."
+    //std::cout << expr_3_fn(3.0) << std::endl;
+
+    // Static-asserts with "Called with wrong number of args."
+    //std::cout << expr_3_fn(3.0, 2.0, 1.0) << std::endl;
+
+    return 0;
+}
+//]