diff --git a/sprout/array/make_array.hpp b/sprout/array/make_array.hpp
index ca6b4ffb..1d6fe2c8 100644
--- a/sprout/array/make_array.hpp
+++ b/sprout/array/make_array.hpp
@@ -25,7 +25,7 @@ namespace sprout {
 	make_array(Types&&... args) {
 		return sprout::array<typename std::remove_cv<T>::type, sizeof...(Types)>{{T(sprout::forward<Types>(args))...}};
 	}
-	// !!!
+	// !!! OLD:
 //	template<typename... Types>
 //	inline SPROUT_CONSTEXPR sprout::array<typename sprout::common_decay<Types...>::type, sizeof...(Types)>
 //	make_array(Types&&... args) {
diff --git a/sprout/checksum/sha1.hpp b/sprout/checksum/sha1.hpp
index 1174e878..887a2cb4 100644
--- a/sprout/checksum/sha1.hpp
+++ b/sprout/checksum/sha1.hpp
@@ -167,7 +167,7 @@ namespace sprout {
 					64,
 					bit_count_ + 64 * 8
 					)
-				// !!!
+				// !!! TEMP:
 //				: process<D + 1>(
 //					h_,
 //					sprout::make_array<std::uint8_t>(args...),
@@ -192,7 +192,7 @@ namespace sprout {
 					bit_count_ + sizeof...(Args) * 8
 					)
 				: block_byte_index_ + sizeof...(Args) == 64
-				// !!!
+				// !!! TEMP:
 //					? process<D + 1>(
 //						h_,
 //						sprout::get_internal(sprout::range::fixed::copy(sprout::make_array<std::uint8_t>(args...), sprout::sub(block_, block_byte_index_))),
diff --git a/sprout/darkroom/objects/polygon/triangle.hpp b/sprout/darkroom/objects/polygon/triangle.hpp
index cf086087..f1d8d7b4 100644
--- a/sprout/darkroom/objects/polygon/triangle.hpp
+++ b/sprout/darkroom/objects/polygon/triangle.hpp
@@ -72,9 +72,9 @@ namespace sprout {
 							? sprout::darkroom::rays::point_of_intersection(ray, distance)
 							: position_type(0, 0, 0)
 							,
-						normal_,//hit_side > 0 ? normal_ : sprout::darkroom::coords::negate(normal_),
+						normal_,// !!! TEMP: hit_side > 0 ? normal_ : sprout::darkroom::coords::negate(normal_),
 						sprout::darkroom::materials::calculate_material(mat_, unit_type(0), unit_type(0)),	// ???
-						false	// !!! is_from_inside
+						false	// !!! TEMP: is_from_inside
 						);
 				}
 				template<typename Ray>
diff --git a/sprout/iterator/const_iterator_cast.hpp b/sprout/iterator/const_iterator_cast.hpp
new file mode 100644
index 00000000..ab48fee4
--- /dev/null
+++ b/sprout/iterator/const_iterator_cast.hpp
@@ -0,0 +1,144 @@
+/*=============================================================================
+  Copyright (c) 2011-2013 Bolero MURAKAMI
+  https://github.com/bolero-MURAKAMI/Sprout
+
+  Distributed under the Boost Software License, Version 1.0. (See accompanying
+  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+=============================================================================*/
+#ifndef SPROUT_ITERATOR_CONST_ITERATOR_CAST_HPP
+#define SPROUT_ITERATOR_CONST_ITERATOR_CAST_HPP
+
+#include <type_traits>
+#include <sprout/config.hpp>
+#include <sprout/type_traits/integral_constant.hpp>
+#include <sprout/type_traits/identity.hpp>
+#include <sprout/type_traits/enabler_if.hpp>
+#include <sprout/adl/not_found.hpp>
+
+namespace sprout {
+	//
+	// is_const_iterator_cast_convertible
+	//
+	template<typename From, typename To>
+	struct is_const_iterator_cast_convertible
+		: public sprout::false_type
+	{};
+	template<typename From, typename To>
+	struct is_const_iterator_cast_convertible<From, To const>
+		: public sprout::is_const_iterator_cast_convertible<From, To>
+	{};
+	template<typename From, typename To>
+	struct is_const_iterator_cast_convertible<From, To volatile>
+		: public sprout::is_const_iterator_cast_convertible<From, To>
+	{};
+	template<typename From, typename To>
+	struct is_const_iterator_cast_convertible<From, To const volatile>
+		: public sprout::is_const_iterator_cast_convertible<From, To>
+	{};
+	template<typename From, typename To>
+	struct is_const_iterator_cast_convertible<From const, To>
+		: public sprout::is_const_iterator_cast_convertible<From, To>
+	{};
+	template<typename From, typename To>
+	struct is_const_iterator_cast_convertible<From const, To const>
+		: public sprout::is_const_iterator_cast_convertible<From, To>
+	{};
+	template<typename From, typename To>
+	struct is_const_iterator_cast_convertible<From const, To volatile>
+		: public sprout::is_const_iterator_cast_convertible<From, To>
+	{};
+	template<typename From, typename To>
+	struct is_const_iterator_cast_convertible<From const, To const volatile>
+		: public sprout::is_const_iterator_cast_convertible<From, To>
+	{};
+	template<typename From, typename To>
+	struct is_const_iterator_cast_convertible<From volatile, To>
+		: public sprout::is_const_iterator_cast_convertible<From, To>
+	{};
+	template<typename From, typename To>
+	struct is_const_iterator_cast_convertible<From volatile, To const>
+		: public sprout::is_const_iterator_cast_convertible<From, To>
+	{};
+	template<typename From, typename To>
+	struct is_const_iterator_cast_convertible<From volatile, To volatile>
+		: public sprout::is_const_iterator_cast_convertible<From, To>
+	{};
+	template<typename From, typename To>
+	struct is_const_iterator_cast_convertible<From volatile, To const volatile>
+		: public sprout::is_const_iterator_cast_convertible<From, To>
+	{};
+	template<typename From, typename To>
+	struct is_const_iterator_cast_convertible<From const volatile, To>
+		: public sprout::is_const_iterator_cast_convertible<From, To>
+	{};
+	template<typename From, typename To>
+	struct is_const_iterator_cast_convertible<From const volatile, To const>
+		: public sprout::is_const_iterator_cast_convertible<From, To>
+	{};
+	template<typename From, typename To>
+	struct is_const_iterator_cast_convertible<From const volatile, To volatile>
+		: public sprout::is_const_iterator_cast_convertible<From, To>
+	{};
+	template<typename From, typename To>
+	struct is_const_iterator_cast_convertible<From const volatile, To const volatile>
+		: public sprout::is_const_iterator_cast_convertible<From, To>
+	{};
+	template<typename From, typename To>
+	struct is_const_iterator_cast_convertible<From*, To*>
+		: public std::is_same<typename std::remove_const<From>::type, typename std::remove_const<To>::type>
+	{};
+
+//	//
+//	// pointer_add_const
+//	//
+//	template<typename T>
+//	struct pointer_add_const
+//		: public std::add_const<T>
+//	{};
+//	template<typename T>
+//	struct pointer_add_const<T*>
+//		: public sprout::identity<typename std::add_const<T>::type*>
+//	{};
+}	// namespace sprout
+
+namespace sprout_adl {
+	template<typename To>
+	sprout::not_found_via_adl const_iterator_conversion(...);
+}	// namespace sprout_adl
+
+namespace sprout {
+	namespace iterator_detail {
+		template<
+			typename T,
+			typename U,
+			typename sprout::enabler_if<sprout::is_const_iterator_cast_convertible<U*, T>::value>::type = sprout::enabler
+		>
+		inline SPROUT_CONSTEXPR T
+		const_iterator_conversion(U* it) {
+			return const_cast<T>(it);
+		}
+	}	// namespace iterator_detail
+}	// namespace sprout
+
+namespace sprout_iterator_detail {
+	template<typename To, typename From>
+	inline SPROUT_CONSTEXPR To
+	const_iterator_cast(From const& it) {
+		using sprout::iterator_detail::const_iterator_conversion;
+		using sprout_adl::const_iterator_conversion;
+		return const_iterator_conversion<To>(it);
+	}
+}	// namespace sprout_iterator_detail
+
+namespace sprout {
+	//
+	// const_iterator_cast
+	//
+	template<typename To, typename From>
+	inline SPROUT_CONSTEXPR To
+	const_iterator_cast(From const& it) {
+		return sprout_iterator_detail::const_iterator_cast<To>(it);
+	}
+}	// namespace sprout
+
+#endif	// #ifndef SPROUT_ITERATOR_CONST_ITERATOR_CAST_HPP
diff --git a/sprout/iterator/index_iterator.hpp b/sprout/iterator/index_iterator.hpp
index 08c2552d..58ce1c3b 100644
--- a/sprout/iterator/index_iterator.hpp
+++ b/sprout/iterator/index_iterator.hpp
@@ -16,10 +16,13 @@
 #include <sprout/iterator/next.hpp>
 #include <sprout/iterator/prev.hpp>
 #include <sprout/iterator/distance.hpp>
+#include <sprout/iterator/const_iterator_cast.hpp>
 #include <sprout/iterator/detail/iterator_to_pointer.hpp>
 #include <sprout/utility/value_holder/value_holder.hpp>
 #include <sprout/utility/swap.hpp>
 #include <sprout/type_traits/integral_constant.hpp>
+#include <sprout/type_traits/identity.hpp>
+#include <sprout/type_traits/enabler_if.hpp>
 
 namespace sprout {
 	//
@@ -283,6 +286,26 @@ namespace sprout {
 	iterator_prev(sprout::index_iterator<Container, C> const& it) {
 		return it.prev();
 	}
+
+	//
+	// is_const_iterator_cast_convertible
+	//
+	template<typename FromContainer, typename ToContainer, bool C>
+	struct is_const_iterator_cast_convertible<sprout::index_iterator<FromContainer, C>, sprout::index_iterator<ToContainer, C> >
+		: public std::is_same<typename std::decay<FromContainer>::type, typename std::decay<ToContainer>::type>
+	{};
+	//
+	// const_iterator_conversion
+	//
+	template<
+		typename T,
+		typename Container, bool C,
+		typename sprout::enabler_if<sprout::is_const_iterator_cast_convertible<sprout::index_iterator<Container, C>, T>::value>::type = sprout::enabler
+	>
+	inline SPROUT_CONSTEXPR T
+	const_iterator_conversion(sprout::index_iterator<Container, C> const& it) {
+		return T(const_cast<typename T::container_type>(it.base()), it.index());
+	}
 }	// namespace sprout
 
 #endif	// #ifndef SPROUT_ITERATOR_INDEX_ITERATOR_HPP
diff --git a/sprout/math/cos.hpp b/sprout/math/cos.hpp
index c1672cdb..cc70a64c 100644
--- a/sprout/math/cos.hpp
+++ b/sprout/math/cos.hpp
@@ -60,7 +60,7 @@ namespace sprout {
 					);
 			}
 
-			// !!!
+			// !!! OLD:
 //			template<typename T>
 //			inline SPROUT_CONSTEXPR T
 //			cos_impl_2(T x) {
diff --git a/sprout/optional/optional.hpp b/sprout/optional/optional.hpp
index 889ff6d4..f177db9d 100644
--- a/sprout/optional/optional.hpp
+++ b/sprout/optional/optional.hpp
@@ -120,7 +120,7 @@ namespace sprout {
 			: init(true)
 			, val(v)
 		{}
-		// !!! for T const&
+		// !!! TEMP: for T const&
 //		SPROUT_CONSTEXPR optional(T&& v)
 //			: init(true)
 //			, val(sprout::move(v))
@@ -145,7 +145,7 @@ namespace sprout {
 			: init(cond)
 			, val(cond ? holder_type(v) : holder_type())
 		{}
-		// !!! for T const&
+		// !!! TEMP: for T const&
 //		SPROUT_CONSTEXPR optional(bool cond, T&& v)
 //			: init(cond)
 //			, val(cond ? holder_type(sprout::move(v)) : holder_type())