diff --git a/sprout/optional.hpp b/sprout/optional.hpp
index 9a96f951..142e2f38 100644
--- a/sprout/optional.hpp
+++ b/sprout/optional.hpp
@@ -1,5 +1,5 @@
 /*=============================================================================
-  Copyright (c) 2011-2017 Bolero MURAKAMI
+  Copyright (c) 2011-2016 Bolero MURAKAMI
   https://github.com/bolero-MURAKAMI/Sprout
 
   Distributed under the Boost Software License, Version 1.0. (See accompanying
@@ -14,10 +14,10 @@
 #include <sprout/optional/io.hpp>
 #include <sprout/optional/nullopt.hpp>
 #include <sprout/optional/in_place.hpp>
+#include <sprout/optional/exceptions.hpp>
 #include <sprout/optional/make_optional.hpp>
 #include <sprout/optional/get.hpp>
 #include <sprout/optional/hash.hpp>
-#include <sprout/optional/exceptions.hpp>
 #include <sprout/optional/container.hpp>
 
 #endif	// #ifndef SPROUT_OPTIONAL_HPP
diff --git a/sprout/utility.hpp b/sprout/utility.hpp
index 79752496..5e46d4d8 100644
--- a/sprout/utility.hpp
+++ b/sprout/utility.hpp
@@ -23,5 +23,6 @@
 #include <sprout/utility/use_default.hpp>
 #include <sprout/utility/loop.hpp>
 #include <sprout/utility/comparison.hpp>
+#include <sprout/utility/fold.hpp>
 
 #endif	// #ifndef SPROUT_UTILITY_HPP
diff --git a/sprout/utility/fold.hpp b/sprout/utility/fold.hpp
new file mode 100644
index 00000000..3afc7b2d
--- /dev/null
+++ b/sprout/utility/fold.hpp
@@ -0,0 +1,98 @@
+/*=============================================================================
+  Copyright (c) 2011-2017 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_UTILITY_FOLD_HPP
+#define SPROUT_UTILITY_FOLD_HPP
+
+#include <sprout/config.hpp>
+#include <sprout/utility/forward.hpp>
+#include <sprout/utility/move.hpp>
+
+namespace sprout {
+	template<typename BinaryOperation>
+	class folder_forwarder {
+	public:
+		typedef BinaryOperation function_type;
+	private:
+		function_type binary_op_;
+	public:
+		explicit SPROUT_CONSTEXPR folder_forwarder(function_type const& binary_op)
+			: binary_op_(binary_op)
+		{}
+		SPROUT_CONSTEXPR function_type const& func() const {
+			return binary_op_;
+		}
+	};
+
+	template<typename BinaryOperation, typename T>
+	class folder {
+	public:
+		typedef BinaryOperation function_type;
+		typedef T value_type;
+	private:
+		function_type binary_op_;
+		value_type value_;
+	public:
+		explicit SPROUT_CONSTEXPR folder(function_type const& binary_op, value_type const& value)
+			: binary_op_(binary_op), value_(value)
+		{}
+		template<typename U>
+		SPROUT_CXX14_CONSTEXPR folder& left(U&& val) {
+			return (value_ = binary_op_(value_, SPROUT_FORWARD(U, val))), *this;
+		}
+		template<typename U>
+		SPROUT_CXX14_CONSTEXPR folder& right(U&& val) {
+			return (value_ = binary_op_(SPROUT_FORWARD(U, val), value_)), *this;
+		}
+		SPROUT_CONSTEXPR function_type const& func() const {
+			return binary_op_;
+		}
+		SPROUT_CONSTEXPR value_type value() const {
+			return value_;
+		}
+		SPROUT_CONSTEXPR value_type get() const {
+			return value_;
+		}
+		SPROUT_CONSTEXPR operator value_type() const {
+			return value_;
+		}
+	};
+
+	template<typename BinaryOperation, typename T>
+	inline SPROUT_CONSTEXPR sprout::folder<BinaryOperation, T>
+	operator,(sprout::folder_forwarder<BinaryOperation> const& lhs, T&& rhs) {
+		return sprout::folder<BinaryOperation, T>(lhs.func(), SPROUT_FORWARD(T, rhs));
+	}
+	template<typename BinaryOperation, typename T>
+	inline SPROUT_CONSTEXPR sprout::folder<BinaryOperation, T>
+	operator,(T&& lhs, sprout::folder_forwarder<BinaryOperation> const& rhs) {
+		return sprout::folder<BinaryOperation, T>(rhs.func(), SPROUT_FORWARD(T, lhs));
+	}
+	template<typename BinaryOperation, typename T, typename U>
+	inline SPROUT_CONSTEXPR sprout::folder<BinaryOperation, T>&&
+	operator,(sprout::folder<BinaryOperation, T>&& lhs, U&& rhs) {
+		return sprout::move(lhs.left(SPROUT_FORWARD(U, rhs)));
+	}
+	template<typename BinaryOperation, typename T, typename U>
+	inline SPROUT_CONSTEXPR sprout::folder<BinaryOperation, T>&&
+	operator,(U&& lhs, sprout::folder<BinaryOperation, T>&& rhs) {
+		return sprout::move(rhs.right(SPROUT_FORWARD(U, lhs)));
+	}
+
+	template<typename BinaryOperation>
+	inline SPROUT_CONSTEXPR sprout::folder_forwarder<BinaryOperation>
+	fold_by(BinaryOperation const& binary_op) {
+		return sprout::folder_forwarder<BinaryOperation>(binary_op);
+	}
+	template<typename BinaryOperation, typename T>
+	inline SPROUT_CONSTEXPR sprout::folder<BinaryOperation, T>
+	fold_by(BinaryOperation const& binary_op, T&& init) {
+		return sprout::folder<BinaryOperation, T>(binary_op, SPROUT_FORWARD(T, init));
+	}
+}	// namespace sprout
+
+#endif	// #ifndef SPROUT_UTILITY_FOLD_HPP
diff --git a/sprout/utility/value_holder.hpp b/sprout/utility/value_holder.hpp
index d98e0ca3..b0198bbf 100644
--- a/sprout/utility/value_holder.hpp
+++ b/sprout/utility/value_holder.hpp
@@ -1,5 +1,5 @@
 /*=============================================================================
-  Copyright (c) 2011-2017 Bolero MURAKAMI
+  Copyright (c) 2011-2016 Bolero MURAKAMI
   https://github.com/bolero-MURAKAMI/Sprout
 
   Distributed under the Boost Software License, Version 1.0. (See accompanying
@@ -10,7 +10,11 @@
 
 #include <sprout/config.hpp>
 #include <sprout/utility/value_holder/value_holder.hpp>
+#include <sprout/utility/value_holder/comparison.hpp>
+#include <sprout/utility/value_holder/io.hpp>
 #include <sprout/utility/value_holder/get.hpp>
 #include <sprout/utility/value_holder/hash.hpp>
+#include <sprout/optional/in_place.hpp>
+#include <sprout/optional/exceptions.hpp>
 
 #endif	// #ifndef SPROUT_UTILITY_VALUE_HOLDER_HPP
diff --git a/sprout/utility/value_holder/comparison.hpp b/sprout/utility/value_holder/comparison.hpp
new file mode 100644
index 00000000..9affc64d
--- /dev/null
+++ b/sprout/utility/value_holder/comparison.hpp
@@ -0,0 +1,56 @@
+/*=============================================================================
+  Copyright (c) 2011-2016 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_UTILITY_VALUE_HOLDER_COMPARISON_HPP
+#define SPROUT_UTILITY_VALUE_HOLDER_COMPARISON_HPP
+
+#include <sprout/config.hpp>
+#include <sprout/utility/compare_pointees.hpp>
+#include <sprout/utility/value_holder/value_holder.hpp>
+
+namespace sprout {
+	//
+	// operator==
+	// operator!=
+	// operator<
+	// operator>
+	// operator<=
+	// operator>=
+	//
+	template<typename T>
+	inline SPROUT_CONSTEXPR bool
+	operator==(sprout::value_holder<T> const& lhs, sprout::value_holder<T> const& rhs) {
+		return sprout::equal_pointees(lhs, rhs);
+	}
+	template<typename T>
+	inline SPROUT_CONSTEXPR bool
+	operator!=(sprout::value_holder<T> const& lhs, sprout::value_holder<T> const& rhs) {
+		return !(lhs == rhs);
+	}
+	template<typename T>
+	inline SPROUT_CONSTEXPR bool
+	operator<(sprout::value_holder<T> const& lhs, sprout::value_holder<T> const& rhs) {
+		return sprout::less_pointees(lhs, rhs);
+	}
+	template<typename T>
+	inline SPROUT_CONSTEXPR bool
+	operator>(sprout::value_holder<T> const& lhs, sprout::value_holder<T> const& rhs) {
+		return rhs < lhs;
+	}
+	template<typename T>
+	inline SPROUT_CONSTEXPR bool
+	operator<=(sprout::value_holder<T> const& lhs, sprout::value_holder<T> const& rhs) {
+		return !(rhs < lhs);
+	}
+	template<typename T>
+	inline SPROUT_CONSTEXPR bool
+	operator>=(sprout::value_holder<T> const& lhs, sprout::value_holder<T> const& rhs) {
+		return !(lhs < rhs);
+	}
+}	// namespace sprout
+
+#endif	// #ifndef SPROUT_UTILITY_VALUE_HOLDER_COMPARISON_HPP
diff --git a/sprout/utility/value_holder/io.hpp b/sprout/utility/value_holder/io.hpp
new file mode 100644
index 00000000..4c80c220
--- /dev/null
+++ b/sprout/utility/value_holder/io.hpp
@@ -0,0 +1,74 @@
+/*=============================================================================
+  Copyright (c) 2011-2016 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_UTILITY_VALUE_HOLDER_IO_HPP
+#define SPROUT_UTILITY_VALUE_HOLDER_IO_HPP
+
+#include <istream>
+#include <ostream>
+#include <sprout/config.hpp>
+#include <sprout/utility/value_holder/value_holder.hpp>
+
+namespace sprout {
+	//
+	// operator>>
+	//
+	template<typename Elem, typename Traits, typename T>
+	inline SPROUT_NON_CONSTEXPR std::basic_istream<Elem, Traits>&
+	operator>>(std::basic_istream<Elem, Traits>& lhs, sprout::value_holder<T>& rhs) {
+		if (lhs.good()) {
+			int d = lhs.get();
+			if (d == ' ') {
+				T x;
+				lhs >> x;
+				rhs = x;
+			} else {
+				lhs.setstate(std::ios::failbit);
+			}
+		}
+		return lhs;
+	}
+	template<typename Elem, typename Traits, typename T>
+	inline SPROUT_NON_CONSTEXPR std::basic_istream<Elem, Traits>&
+	operator>>(std::basic_istream<Elem, Traits>& lhs, sprout::value_holder<T&>& rhs) {
+		if (lhs.good()) {
+			int d = lhs.get();
+			if (d == ' ') {
+				T x;
+				lhs >> x;
+				rhs = x;
+			} else {
+				if (d == '-') {
+					d = lhs.get();
+					if (d == '-') {
+						rhs = sprout::value_holder<T&>();
+						return lhs;
+					}
+				}
+				lhs.setstate(std::ios::failbit);
+			}
+		}
+		return lhs;
+	}
+	//
+	// operator<<
+	//
+	template<typename Elem, typename Traits, typename T>
+	inline SPROUT_NON_CONSTEXPR std::basic_ostream<Elem, Traits>&
+	operator<<(std::basic_ostream<Elem, Traits>& lhs, sprout::value_holder<T> const& rhs) {
+		if (lhs.good()) {
+			if (!rhs) {
+				lhs << "--";
+			} else {
+				lhs << ' ' << *rhs;
+			}
+		}
+		return lhs;
+	}
+}	// namespace sprout
+
+#endif	// #ifndef SPROUT_UTILITY_VALUE_HOLDER_IO_HPP
diff --git a/sprout/utility/value_holder/value_holder.hpp b/sprout/utility/value_holder/value_holder.hpp
index 9aaf2b12..0e09b6d8 100644
--- a/sprout/utility/value_holder/value_holder.hpp
+++ b/sprout/utility/value_holder/value_holder.hpp
@@ -1,5 +1,5 @@
 /*=============================================================================
-  Copyright (c) 2011-2017 Bolero MURAKAMI
+  Copyright (c) 2011-2016 Bolero MURAKAMI
   https://github.com/bolero-MURAKAMI/Sprout
 
   Distributed under the Boost Software License, Version 1.0. (See accompanying
@@ -11,12 +11,14 @@
 #include <type_traits>
 #include <initializer_list>
 #include <sprout/config.hpp>
+#include <sprout/workaround/std/cstddef.hpp>
 #include <sprout/memory/addressof.hpp>
 #include <sprout/utility/swap.hpp>
 #include <sprout/utility/forward.hpp>
 #include <sprout/utility/move.hpp>
 #include <sprout/type_traits/is_constructible.hpp>
 #include <sprout/optional/in_place.hpp>
+#include <sprout/optional/exceptions.hpp>
 
 namespace sprout {
 	namespace detail {
@@ -44,10 +46,10 @@ namespace sprout {
 			static SPROUT_CONSTEXPR holder_type&& hold(movable_param_type p) SPROUT_NOEXCEPT {
 				return sprout::move(p);
 			}
-			static SPROUT_CONSTEXPR reference ref(holder_type& r) {
+			static SPROUT_CONSTEXPR reference ref(holder_type& r) SPROUT_NOEXCEPT {
 				return r;
 			}
-			static SPROUT_CONSTEXPR const_reference ref(holder_type const& r) {
+			static SPROUT_CONSTEXPR const_reference ref(holder_type const& r) SPROUT_NOEXCEPT {
 				return r;
 			}
 			static SPROUT_CONSTEXPR pointer ptr(holder_type& r) SPROUT_NOEXCEPT {
@@ -81,11 +83,11 @@ namespace sprout {
 			static SPROUT_CONSTEXPR holder_type const&& hold(movable_param_type p) SPROUT_NOEXCEPT {
 				return sprout::move(p);
 			}
-			static SPROUT_CONSTEXPR reference ref(holder_type& r) {
-				return *r;
+			static SPROUT_CONSTEXPR reference ref(holder_type& r) SPROUT_NOEXCEPT {
+				return r;
 			}
-			static SPROUT_CONSTEXPR const_reference ref(holder_type const& r) {
-				return *r;
+			static SPROUT_CONSTEXPR const_reference ref(holder_type const& r) SPROUT_NOEXCEPT {
+				return r;
 			}
 			static SPROUT_CONSTEXPR pointer ptr(holder_type& r) SPROUT_NOEXCEPT {
 				return sprout::addressof(r);
@@ -116,7 +118,9 @@ namespace sprout {
 				return sprout::addressof(p);
 			}
 			static SPROUT_CONSTEXPR reference ref(holder_type r) {
-				return *r;
+				return r ? *r
+					: (throw sprout::bad_optional_access("value_holder<>: bad optional access"), *r)
+					;
 			}
 			static SPROUT_CONSTEXPR pointer ptr(holder_type r) SPROUT_NOEXCEPT {
 				return r;
@@ -144,7 +148,9 @@ namespace sprout {
 				return sprout::addressof(p);
 			}
 			static SPROUT_CONSTEXPR reference ref(holder_type r) {
-				return *r;
+				return r ? *r
+					: (throw sprout::bad_optional_access("value_holder<>: bad optional access"), *r)
+					;
 			}
 			static SPROUT_CONSTEXPR pointer ptr(holder_type r) SPROUT_NOEXCEPT {
 				return r;
@@ -306,6 +312,13 @@ namespace sprout {
 		SPROUT_CONSTEXPR mutable_or_const_pointer get_ptr() const SPROUT_NOEXCEPT {
 			return get_pointer();
 		}
+
+		SPROUT_EXPLICIT_CONVERSION SPROUT_CONSTEXPR operator bool() const SPROUT_NOEXCEPT {
+			return is_initialized();
+		}
+		SPROUT_CONSTEXPR bool operator!() const SPROUT_NOEXCEPT {
+			return !is_initialized();
+		}
 		SPROUT_CONSTEXPR bool is_initialized() const SPROUT_NOEXCEPT {
 			return !!get_pointer();
 		}