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+/*!
+@file
+Forward declares `boost::hana::prefix`.
+
+@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_PREFIX_HPP
+#define BOOST_HANA_FWD_PREFIX_HPP
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/when.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! Inserts a value before each element of a monadic structure.
+    //! @ingroup group-MonadPlus
+    //!
+    //! Given a monadic structure `xs` and a value `z` called the prefix,
+    //! `prefix` returns a new monadic structure. `prefix` satisfies
+    //! @code
+    //!     prefix(xs, z) == flatten(transform(xs, [](auto x) {
+    //!         return concat(lift<M>(z), lift<M>(x));
+    //!     }))
+    //! @endcode
+    //!
+    //! For sequences, this simply corresponds to inserting the prefix before
+    //! each element of the sequence. For example, given a sequence
+    //! `[x1, ..., xn]`, `prefix` will return
+    //! @code
+    //!     [z, x1, z, x2, ..., z, xn]
+    //! @endcode
+    //! As explained above, this can be generalized to other MonadPlus models,
+    //! with various levels of interest.
+    //!
+    //!
+    //! Signature
+    //! ---------
+    //! Given a MonadPlus `M`, the signature is
+    //! @f$ \mathrm{prefix} : M(T) \times T \to M(T) @f$.
+    //!
+    //! @param xs
+    //! A monadic structure.
+    //!
+    //! @param pref
+    //! A value (the prefix) to insert before each element of a monadic
+    //! structure.
+    //!
+    //!
+    //! Example
+    //! -------
+    //! @include example/prefix.cpp
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+    constexpr auto prefix = [](auto&& xs, auto&& pref) {
+        return tag-dispatched;
+    };
+#else
+    template <typename M, typename = void>
+    struct prefix_impl : prefix_impl<M, when<true>> { };
+
+    struct prefix_t {
+        template <typename Xs, typename Pref>
+        constexpr auto operator()(Xs&& xs, Pref&& pref) const;
+    };
+
+    constexpr prefix_t prefix{};
+#endif
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FWD_PREFIX_HPP