diff --git a/sprout/index_tuple/tuple.hpp b/sprout/index_tuple/tuple.hpp
index f7dd15e0..417d2197 100644
--- a/sprout/index_tuple/tuple.hpp
+++ b/sprout/index_tuple/tuple.hpp
@@ -35,7 +35,7 @@ namespace std {
 	struct tuple_element<I, sprout::integer_sequence<T, Is...> > {
 		static_assert(I < sizeof...(Is), "tuple_element<>: index out of range");
 	public:
-		typedef typename sprout::tppack_c_at<I, T, Is...>::type type;
+		typedef typename sprout::pack_element_c<I, T, Is...>::type type;
 	};
 
 #if !SPROUT_USE_TEMPLATE_ALIASES
diff --git a/sprout/optional/optional.hpp b/sprout/optional/optional.hpp
index bef97967..889ff6d4 100644
--- a/sprout/optional/optional.hpp
+++ b/sprout/optional/optional.hpp
@@ -17,6 +17,7 @@
 #include <sprout/utility/swap.hpp>
 #include <sprout/utility/forward.hpp>
 #include <sprout/utility/move.hpp>
+#include <sprout/type_traits/std_type_traits.hpp>
 #include <sprout/type_traits/is_convert_constructible.hpp>
 #include <sprout/none.hpp>
 #include <sprout/optional/nullopt.hpp>
@@ -53,7 +54,7 @@ namespace sprout {
 	public:
 		template<typename... Args>
 		struct is_constructible_args
-			: public std::is_constructible<T, Args&&...>
+			: public sprout::is_constructible<T, Args&&...>
 		{};
 	public:
 		static SPROUT_CONSTEXPR reference_type get(optional& t) SPROUT_NOEXCEPT {
diff --git a/sprout/tuple/tuple/tuple_decl.hpp b/sprout/tuple/tuple/tuple_decl.hpp
index ca5e1ba1..a5e9fe38 100644
--- a/sprout/tuple/tuple/tuple_decl.hpp
+++ b/sprout/tuple/tuple/tuple_decl.hpp
@@ -267,8 +267,8 @@ namespace sprout {
 			struct is_flexibly_convert_constructible_impl<sprout::index_tuple<Indexes...>, Utypes...>
 				: public sprout::tpp::all_of<
 					sprout::is_convert_constructible<
-						typename sprout::tppack_at<Indexes, Types...>::type,
-						typename sprout::tppack_at<Indexes, Utypes...>::type
+						typename sprout::pack_element<Indexes, Types...>::type,
+						typename sprout::pack_element<Indexes, Utypes...>::type
 					>...
 				>
 			{};
@@ -466,20 +466,6 @@ namespace sprout {
 		{
 			lhs.swap(rhs);
 		}
-
-		namespace detail {
-			template<std::size_t I, typename T>
-			struct tuple_element_impl;
-			template<typename Head, typename... Tail>
-			struct tuple_element_impl<0, sprout::tuples::tuple<Head, Tail...> > {
-			public:
-				typedef Head type;
-			};
-			template<std::size_t I, typename Head, typename... Tail>
-			struct tuple_element_impl<I, sprout::tuples::tuple<Head, Tail...> >
-				: public sprout::tuples::detail::tuple_element_impl<I - 1, sprout::tuples::tuple<Tail...> >
-			{};
-		}	// namespace detail
 	}	// namespace tuples
 
 	using sprout::tuples::tuple;
@@ -504,7 +490,7 @@ namespace std {
 	//
 	template<std::size_t I, typename... Types>
 	struct tuple_element<I, sprout::tuples::tuple<Types...> >
-		: public sprout::tuples::detail::tuple_element_impl<I, sprout::tuples::tuple<Types...> >
+		: public sprout::pack_element<I, Types...>
 	{};
 #if defined(__clang__)
 #	pragma clang diagnostic pop
diff --git a/sprout/tuple/tuple/tuple_size.hpp b/sprout/tuple/tuple/tuple_size.hpp
index eb9dc7b8..0c5fd843 100644
--- a/sprout/tuple/tuple/tuple_size.hpp
+++ b/sprout/tuple/tuple/tuple_size.hpp
@@ -10,6 +10,7 @@
 
 #include <tuple>
 #include <sprout/config.hpp>
+#include <sprout/type_traits/detail/type_traits_wrapper.hpp>
 
 namespace sprout {
 	namespace tuples {
@@ -18,7 +19,7 @@ namespace sprout {
 		//
 		template<typename T>
 		struct tuple_size
-			: public std::tuple_size<T>
+			: public sprout::detail::type_traits_wrapper<std::tuple_size<T> >
 		{};
 		template<typename T>
 		struct tuple_size<T const>
diff --git a/sprout/type/tuple.hpp b/sprout/type/tuple.hpp
index debc2287..290e63c6 100644
--- a/sprout/type/tuple.hpp
+++ b/sprout/type/tuple.hpp
@@ -11,6 +11,7 @@
 #include <cstddef>
 #include <tuple>
 #include <sprout/config.hpp>
+#include <sprout/type_traits/detail/type_traits_wrapper.hpp>
 
 namespace sprout {
 	namespace types {
@@ -34,7 +35,7 @@ namespace sprout {
 		//
 		template<typename T, typename Enable = void>
 		struct tuple_size
-			: public std::tuple_size<T>
+			: public sprout::detail::type_traits_wrapper<std::tuple_size<T> >
 		{};
 		//
 		// tuple_element
diff --git a/sprout/type/type_tuple.hpp b/sprout/type/type_tuple.hpp
index eda80fb2..c1a35962 100644
--- a/sprout/type/type_tuple.hpp
+++ b/sprout/type/type_tuple.hpp
@@ -10,12 +10,14 @@
 
 #include <cstddef>
 #include <tuple>
+#include <type_traits>
 #include <sprout/config.hpp>
 #include <sprout/type/tuple.hpp>
 #include <sprout/type/iterator.hpp>
 #include <sprout/type/iterator/index_iterator.hpp>
 #include <sprout/type/rebind_types.hpp>
 #include <sprout/type_traits/integral_constant.hpp>
+#include <sprout/type_traits/identity.hpp>
 
 namespace sprout {
 	namespace types {
@@ -48,16 +50,41 @@ namespace sprout {
 		};
 
 		namespace detail {
-			template<std::size_t I, typename T>
-			struct tuple_element_impl;
+			template<typename T>
+			struct tuple_head;
 			template<typename Head, typename... Tail>
-			struct tuple_element_impl<0, sprout::types::type_tuple<Head, Tail...> > {
-			public:
-				typedef Head type;
-			};
-			template<std::size_t I, typename Head, typename... Tail>
-			struct tuple_element_impl<I, sprout::types::type_tuple<Head, Tail...> >
-				: public sprout::types::detail::tuple_element_impl<I - 1, sprout::types::type_tuple<Tail...> >
+			struct tuple_head<sprout::types::type_tuple<Head, Tail...> >
+				: public sprout::identity<Head>
+			{};
+
+			template<std::size_t I, std::size_t N, typename T, typename Enable = void>
+			struct tuple_skip;
+			template<std::size_t I, std::size_t N, typename Head, typename... Tail>
+			struct tuple_skip<
+				I, N, sprout::types::type_tuple<Head, Tail...>,
+				typename std::enable_if<(I == 0)>::type
+			>
+				: public sprout::identity<sprout::types::type_tuple<Head, Tail...> >
+			{};
+			template<std::size_t I, std::size_t N, typename Head, typename... Tail>
+			struct tuple_skip<
+				I, N, sprout::types::type_tuple<Head, Tail...>,
+				typename std::enable_if<(I != 0 && I < N / 2)>::type
+			>
+				: public sprout::types::detail::tuple_skip<
+					I - 1, N / 2 - 1,
+					sprout::types::type_tuple<Tail...>
+				>
+			{};
+			template<std::size_t I, std::size_t N, typename Head, typename... Tail>
+			struct tuple_skip<
+				I, N, sprout::types::type_tuple<Head, Tail...>,
+				typename std::enable_if<(I != 0 && I >= N / 2)>::type
+			>
+				: public sprout::types::detail::tuple_skip<
+					I - N / 2, N - N / 2,
+					typename sprout::types::detail::tuple_skip<N / 2 - 1, N / 2, sprout::types::type_tuple<Tail...> >::type
+				>
 			{};
 		}	// namespace detail
 	}	// namespace types
@@ -84,7 +111,9 @@ namespace std {
 	//
 	template<std::size_t I, typename... Types>
 	struct tuple_element<I, sprout::types::type_tuple<Types...> >
-		: public sprout::types::detail::tuple_element_impl<I, sprout::types::type_tuple<Types...> >
+		: public sprout::types::detail::tuple_head<
+			typename sprout::types::detail::tuple_skip<I, sizeof...(Types), sprout::types::type_tuple<Types...> >::type
+		>
 	{};
 #if defined(__clang__)
 #	pragma clang diagnostic pop
diff --git a/sprout/type_traits/integral_constant.hpp b/sprout/type_traits/integral_constant.hpp
index 47ee3cdd..96a05660 100644
--- a/sprout/type_traits/integral_constant.hpp
+++ b/sprout/type_traits/integral_constant.hpp
@@ -27,6 +27,10 @@ namespace sprout {
 		operator value_type() const SPROUT_NOEXCEPT {
 			return std::integral_constant<T, v>::value;
 		}
+		SPROUT_CONSTEXPR bool
+		operator!() const SPROUT_NOEXCEPT {
+			return !std::integral_constant<T, v>::value;
+		}
 		SPROUT_CONSTEXPR value_type
 		operator()() const SPROUT_NOEXCEPT {
 			return std::integral_constant<T, v>::value;
diff --git a/sprout/utility/pack.hpp b/sprout/utility/pack.hpp
index 44b0e390..5df9ef2d 100644
--- a/sprout/utility/pack.hpp
+++ b/sprout/utility/pack.hpp
@@ -11,45 +11,29 @@
 #include <cstddef>
 #include <sprout/config.hpp>
 #include <sprout/utility/forward.hpp>
+#include <sprout/type/type_tuple.hpp>
 #include <sprout/type_traits/integral_constant.hpp>
 #include <sprout/type_traits/enabler_if.hpp>
 
 namespace sprout {
 	//
-	// tppack_at
+	// pack_element
 	//
-	namespace detail {
-		template<std::size_t I, typename Head, typename... Tail>
-		struct tppack_at_impl_1
-			: public sprout::detail::tppack_at_impl_1<I - 1, Tail...>
-		{};
-		template<typename Head, typename... Tail>
-		struct tppack_at_impl_1<0, Head, Tail...> {
-		public:
-			typedef Head type;
-		};
-		template<std::size_t I, typename... Args>
-		struct tppack_at_impl
-			: public sprout::detail::tppack_at_impl_1<I, Args...>
-		{
-			static_assert(I < sizeof...(Args), "I < sizeof...(Args)");
-		};
-	}	// namespace detail
 	template<std::size_t I, typename... Args>
-	struct tppack_at
-		: public sprout::detail::tppack_at_impl<I, Args...>
+	struct pack_element
+		: public std::tuple_element<I, sprout::types::type_tuple<Args...> >
 	{};
 
 	//
-	// tppack_c_at
+	// pack_element_c
 	//
 	template<std::size_t I, typename T, T... Args>
-	struct tppack_c_at
-		: public sprout::tppack_at<I, sprout::integral_constant<T, Args>...>::type
+	struct pack_element_c
+		: public sprout::pack_element<I, sprout::integral_constant<T, Args>...>::type
 	{};
 
 	//
-	// fppack_at
+	// pack_get
 	//
 	namespace detail {
 		template<
@@ -57,7 +41,7 @@ namespace sprout {
 			typename sprout::enabler_if<I == 0>::type = sprout::enabler
 		>
 		inline SPROUT_CONSTEXPR R
-		fppack_at_impl(Head&& head, Tail&&...) {
+		pack_get_impl(Head&& head, Tail&&...) {
 			return sprout::forward<Head>(head);
 		}
 		template<
@@ -65,16 +49,16 @@ namespace sprout {
 			typename sprout::enabler_if<I != 0>::type = sprout::enabler
 		>
 		inline SPROUT_CONSTEXPR R
-		fppack_at_impl(Head&&, Tail&&... tail) {
-			return sprout::detail::fppack_at_impl<I - 1, R>(sprout::forward<Tail>(tail)...);
+		pack_get_impl(Head&&, Tail&&... tail) {
+			return sprout::detail::pack_get_impl<I - 1, R>(sprout::forward<Tail>(tail)...);
 		}
 	}	// namespace detail
 	template<std::size_t I, typename... Args>
-	inline SPROUT_CONSTEXPR typename sprout::tppack_at<I, Args&&...>::type
-	fppack_at(Args&&... args) {
-		return sprout::detail::fppack_at_impl<
+	inline SPROUT_CONSTEXPR typename sprout::pack_element<I, Args&&...>::type
+	pack_get(Args&&... args) {
+		return sprout::detail::pack_get_impl<
 			I,
-			typename sprout::tppack_at<I, Args&&...>::type
+			typename sprout::pack_element<I, Args&&...>::type
 		>(sprout::forward<Args>(args)...);
 	}
 }	// namespace sprout
diff --git a/sprout/utility/value_holder/value_holder.hpp b/sprout/utility/value_holder/value_holder.hpp
index 245d2718..80b41d27 100644
--- a/sprout/utility/value_holder/value_holder.hpp
+++ b/sprout/utility/value_holder/value_holder.hpp
@@ -14,6 +14,7 @@
 #include <sprout/utility/swap.hpp>
 #include <sprout/utility/forward.hpp>
 #include <sprout/utility/move.hpp>
+#include <sprout/type_traits/std_type_traits.hpp>
 #include <sprout/optional/in_place.hpp>
 
 namespace sprout {
@@ -184,7 +185,7 @@ namespace sprout {
 	public:
 		template<typename... Args>
 		struct is_constructible_args
-			: public std::is_constructible<T, Args&&...>
+			: public sprout::is_constructible<T, Args&&...>
 		{};
 	public:
 		static SPROUT_CONSTEXPR reference get(value_holder& t) SPROUT_NOEXCEPT {