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diff --git a/boost/hana/concat.hpp b/boost/hana/concat.hpp
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+/*!
+@file
+Defines `boost::hana::concat`.
+
+@copyright Louis Dionne 2013-2016
+Distributed under the Boost Software License, Version 1.0.
+(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
+ */
+
+#ifndef BOOST_HANA_CONCAT_HPP
+#define BOOST_HANA_CONCAT_HPP
+
+#include <boost/hana/fwd/concat.hpp>
+
+#include <boost/hana/at.hpp>
+#include <boost/hana/concept/monad_plus.hpp>
+#include <boost/hana/concept/sequence.hpp>
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/dispatch.hpp>
+#include <boost/hana/core/make.hpp>
+#include <boost/hana/length.hpp>
+
+#include <cstddef>
+#include <type_traits>
+#include <utility>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! @cond
+    template <typename Xs, typename Ys>
+    constexpr auto concat_t::operator()(Xs&& xs, Ys&& ys) const {
+        using M = typename hana::tag_of<Xs>::type;
+        using Concat = BOOST_HANA_DISPATCH_IF(concat_impl<M>,
+            hana::MonadPlus<M>::value &&
+            std::is_same<typename hana::tag_of<Ys>::type, M>::value
+        );
+
+    #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
+        static_assert(std::is_same<typename hana::tag_of<Ys>::type, M>::value,
+        "hana::concat(xs, ys) requires 'xs' and 'ys' to have the same tag");
+
+        static_assert(hana::MonadPlus<M>::value,
+        "hana::concat(xs, ys) requires 'xs' and 'ys' to be MonadPlus");
+    #endif
+
+        return Concat::apply(static_cast<Xs&&>(xs), static_cast<Ys&&>(ys));
+    }
+    //! @endcond
+
+    template <typename M, bool condition>
+    struct concat_impl<M, when<condition>> : default_ {
+        template <typename ...Args>
+        static constexpr auto apply(Args&& ...) = delete;
+    };
+
+    template <typename S>
+    struct concat_impl<S, when<Sequence<S>::value>> {
+        template <typename Xs, typename Ys, std::size_t ...xi, std::size_t ...yi>
+        static constexpr auto
+        concat_helper(Xs&& xs, Ys&& ys, std::index_sequence<xi...>,
+                                        std::index_sequence<yi...>)
+        {
+            return hana::make<S>(
+                hana::at_c<xi>(static_cast<Xs&&>(xs))...,
+                hana::at_c<yi>(static_cast<Ys&&>(ys))...
+            );
+        }
+
+        template <typename Xs, typename Ys>
+        static constexpr auto apply(Xs&& xs, Ys&& ys) {
+            constexpr std::size_t xi = decltype(hana::length(xs))::value;
+            constexpr std::size_t yi = decltype(hana::length(ys))::value;
+            return concat_helper(static_cast<Xs&&>(xs), static_cast<Ys&&>(ys),
+                                 std::make_index_sequence<xi>{},
+                                 std::make_index_sequence<yi>{});
+        }
+    };
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_CONCAT_HPP