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diff --git a/boost/hana/keys.hpp b/boost/hana/keys.hpp
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+/*!
+@file
+Defines `boost::hana::keys`.
+
+@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_KEYS_HPP
+#define BOOST_HANA_KEYS_HPP
+
+#include <boost/hana/fwd/keys.hpp>
+
+#include <boost/hana/accessors.hpp>
+#include <boost/hana/concept/struct.hpp>
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/dispatch.hpp>
+#include <boost/hana/first.hpp>
+#include <boost/hana/transform.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! @cond
+    template <typename Map>
+    constexpr auto keys_t::operator()(Map&& map) const {
+        return keys_impl<typename hana::tag_of<Map>::type>::apply(
+            static_cast<Map&&>(map)
+        );
+    }
+    //! @endcond
+
+    template <typename T, bool condition>
+    struct keys_impl<T, when<condition>> : default_ {
+        template <typename ...Args>
+        static constexpr auto apply(Args&& ...) = delete;
+    };
+
+    template <typename S>
+    struct keys_impl<S, when<hana::Struct<S>::value>> {
+        template <typename Object>
+        static constexpr auto apply(Object const&) {
+            return hana::transform(hana::accessors<S>(), hana::first);
+        }
+    };
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_KEYS_HPP