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+/*!
+@file
+Forward declares `boost::hana::contains` and `boost::hana::in`.
+
+@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_CONTAINS_HPP
+#define BOOST_HANA_FWD_CONTAINS_HPP
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/when.hpp>
+#include <boost/hana/functional/flip.hpp>
+#include <boost/hana/functional/infix.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+ //! Returns whether the key occurs in the structure.
+ //! @ingroup group-Searchable
+ //!
+ //! Given a `Searchable` structure `xs` and a `key`, `contains` returns
+ //! whether any of the keys of the structure is equal to the given `key`.
+ //! If the structure is not finite, an equal key has to appear at a finite
+ //! position in the structure for this method to finish. For convenience,
+ //! `contains` can also be applied in infix notation.
+ //!
+ //!
+ //! @param xs
+ //! The structure to search.
+ //!
+ //! @param key
+ //! A key to be searched for in the structure. The key has to be
+ //! `Comparable` with the other keys of the structure.
+ //!
+ //!
+ //! Example
+ //! -------
+ //! @include example/contains.cpp
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+ constexpr auto contains = [](auto&& xs, auto&& key) {
+ return tag-dispatched;
+ };
+#else
+ template <typename S, typename = void>
+ struct contains_impl : contains_impl<S, when<true>> { };
+
+ struct contains_t {
+ template <typename Xs, typename Key>
+ constexpr auto operator()(Xs&& xs, Key&& key) const;
+ };
+
+ constexpr auto contains = hana::infix(contains_t{});
+#endif
+
+ //! Return whether the key occurs in the structure.
+ //! @ingroup group-Searchable
+ //!
+ //! Specifically, this is equivalent to `contains`, except `in` takes its
+ //! arguments in reverse order. Like `contains`, `in` can also be applied
+ //! in infix notation for increased expressiveness. This function is not a
+ //! method that can be overriden; it is just a convenience function
+ //! provided with the concept.
+ //!
+ //!
+ //! Example
+ //! -------
+ //! @include example/in.cpp
+ constexpr auto in = hana::infix(hana::flip(hana::contains));
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FWD_CONTAINS_HPP