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+/*!
+@file
+Forward declares `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_FWD_CONCAT_HPP
+#define BOOST_HANA_FWD_CONCAT_HPP
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/when.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! Combine two monadic structures together.
+    //! @ingroup group-MonadPlus
+    //!
+    //! Given two monadic structures, `concat` combines them together and
+    //! returns a new monadic structure. The exact definition of `concat`
+    //! will depend on the exact model of MonadPlus at hand, but for
+    //! sequences it corresponds intuitively to simple concatenation.
+    //!
+    //! Also note that combination is not required to be commutative.
+    //! In other words, there is no requirement that
+    //! @code
+    //!     concat(xs, ys) == concat(ys, xs)
+    //! @endcode
+    //! and indeed it does not hold in general.
+    //!
+    //!
+    //! Signature
+    //! ---------
+    //! Given a `MonadPlus` `M`, the signature of `concat` is
+    //! @f$ \mathtt{concat} : M(T) \times M(T) \to M(T) @f$.
+    //!
+    //! @param xs, ys
+    //! Two monadic structures to combine together.
+    //!
+    //!
+    //! Example
+    //! -------
+    //! @include example/concat.cpp
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+    constexpr auto concat = [](auto&& xs, auto&& ys) {
+        return tag-dispatched;
+    };
+#else
+    template <typename M, typename = void>
+    struct concat_impl : concat_impl<M, when<true>> { };
+
+    struct concat_t {
+        template <typename Xs, typename Ys>
+        constexpr auto operator()(Xs&& xs, Ys&& ys) const;
+    };
+
+    constexpr concat_t concat{};
+#endif
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FWD_CONCAT_HPP