diff --git a/sprout/string/string.hpp b/sprout/string/string.hpp
index 21f06abe..ac98b390 100644
--- a/sprout/string/string.hpp
+++ b/sprout/string/string.hpp
@@ -28,6 +28,7 @@
 #include <sprout/algorithm/cxx14/swap_ranges.hpp>
 #include <sprout/utility/forward.hpp>
 #include <sprout/utility/swap.hpp>
+#include <sprout/utility/string_view/string_view_fwd.hpp>
 #include <sprout/type_traits/integral_constant.hpp>
 #include <sprout/type_traits/identity.hpp>
 #include <sprout/type_traits/enabler_if.hpp>
@@ -885,6 +886,9 @@ namespace sprout {
 		SPROUT_EXPLICIT_CONVERSION SPROUT_NON_CONSTEXPR operator std::basic_string<T, Traits, Allocator>() const {
 			return std::basic_string<T, Traits, Allocator>(data(), size());
 		}
+		operator sprout::basic_string_view<T, Traits>() const SPROUT_NOEXCEPT {
+			return sprout::basic_string_view<T, Traits>(*this);
+		}
 
 		SPROUT_CXX14_CONSTEXPR void rangecheck(size_type i) const {
 			return i >= size() ? throw std::out_of_range("basic_string<>: index out of range")
diff --git a/sprout/sub_array/sub.hpp b/sprout/sub_array/sub.hpp
index 33b04bd5..eefce31d 100644
--- a/sprout/sub_array/sub.hpp
+++ b/sprout/sub_array/sub.hpp
@@ -137,6 +137,12 @@ namespace sprout {
 		return sprout::sub_window(arr, to_first);
 	}
 
+	template<typename Container>
+	inline SPROUT_CONSTEXPR typename std::enable_if<!sprout::is_sub_array<Container>::value, sprout::sub_array<Container&> >::type
+	sub(Container& arr) {
+		return sprout::sub(arr, sprout::begin(arr), sprout::end(arr));
+	}
+
 	//
 	// csub
 	//
@@ -218,6 +224,12 @@ namespace sprout {
 		return sprout::csub_window(arr, to_first);
 	}
 
+	template<typename Container>
+	inline SPROUT_CONSTEXPR typename std::enable_if<!sprout::is_sub_array<Container>::value, sprout::sub_array<Container const&> >::type
+	csub(Container const& arr) {
+		return sprout::csub(arr, sprout::begin(arr), sprout::end(arr));
+	}
+
 	//
 	// sub_copy
 	//
@@ -298,6 +310,12 @@ namespace sprout {
 	{
 		return sprout::sub_window_copy(arr, to_first);
 	}
+
+	template<typename Container>
+	inline SPROUT_CONSTEXPR typename std::enable_if<!sprout::is_sub_array<Container>::value, sprout::sub_array<Container> >::type
+	sub_copy(Container const& arr) {
+		return sprout::sub_copy(arr, sprout::begin(arr), sprout::end(arr));
+	}
 }	// namespace sprout
 
 #endif	// #ifndef SPROUT_SUB_ARRAY_SUB_HPP
diff --git a/sprout/utility/string_view/string_view.hpp b/sprout/utility/string_view/string_view.hpp
index 7cd8fb97..3f6dfba4 100644
--- a/sprout/utility/string_view/string_view.hpp
+++ b/sprout/utility/string_view/string_view.hpp
@@ -17,7 +17,7 @@
 #include <sprout/iterator/reverse_iterator.hpp>
 #include <sprout/iterator/operation.hpp>
 #include <sprout/utility/swap.hpp>
-#include <sprout/string/char_traits.hpp>
+#include <sprout/utility/string_view/string_view_fwd.hpp>
 #include <sprout/string/npos.hpp>
 #include <sprout/string/string.hpp>
 #include <sprout/string/detail/operations.hpp>
@@ -31,7 +31,7 @@ namespace sprout {
 	//
 	// basic_string_view
 	//
-	template<typename T, typename Traits = sprout::char_traits<T> >
+	template<typename T, typename Traits>
 	class basic_string_view {
 	public:
 		typedef T value_type;
@@ -94,14 +94,14 @@ namespace sprout {
 			: ptr_(str), len_(traits_type::length(str))
 		{}
 		template<std::size_t N>
-		SPROUT_CONSTEXPR basic_string_view(sprout::basic_string<T, N, Traits> const& str)
+		SPROUT_CONSTEXPR basic_string_view(sprout::basic_string<T, N, Traits> const& str) SPROUT_NOEXCEPT
 			: ptr_(str.data()), len_(str.size())
 		{}
 		template<typename Allocator>
-		SPROUT_NON_CONSTEXPR basic_string_view(std::basic_string<T, Traits, Allocator> const& str)
+		SPROUT_NON_CONSTEXPR basic_string_view(std::basic_string<T, Traits, Allocator> const& str) SPROUT_NOEXCEPT
 			: ptr_(str.data()), len_(str.size())
 		{}
-		SPROUT_CONSTEXPR basic_string_view(const_pointer str, size_type len)
+		SPROUT_CONSTEXPR basic_string_view(const_pointer str, size_type len) SPROUT_NOEXCEPT
 			: ptr_(str), len_(len)
 		{}
 		// iterators:
diff --git a/sprout/utility/string_view/string_view_fwd.hpp b/sprout/utility/string_view/string_view_fwd.hpp
new file mode 100644
index 00000000..288bfb39
--- /dev/null
+++ b/sprout/utility/string_view/string_view_fwd.hpp
@@ -0,0 +1,22 @@
+/*=============================================================================
+  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_STRING_VIEW_STRING_VIEW_FWD_HPP
+#define SPROUT_UTILITY_STRING_VIEW_STRING_VIEW_FWD_HPP
+
+#include <sprout/config.hpp>
+#include <sprout/string/char_traits.hpp>
+
+namespace sprout {
+	//
+	// basic_string_view
+	//
+	template<typename T, typename Traits = sprout::char_traits<T> >
+	class basic_string_view;
+}	// namespace sprout
+
+#endif	// #ifndef SPROUT_UTILITY_STRING_VIEW_STRING_VIEW_FWD_HPP
diff --git a/sprout/utility/value_holder/value_holder.hpp b/sprout/utility/value_holder/value_holder.hpp
index 0e09b6d8..10ece355 100644
--- a/sprout/utility/value_holder/value_holder.hpp
+++ b/sprout/utility/value_holder/value_holder.hpp
@@ -118,9 +118,11 @@ namespace sprout {
 				return sprout::addressof(p);
 			}
 			static SPROUT_CONSTEXPR reference ref(holder_type r) {
-				return r ? *r
-					: (throw sprout::bad_optional_access("value_holder<>: bad optional access"), *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;
@@ -148,9 +150,11 @@ namespace sprout {
 				return sprout::addressof(p);
 			}
 			static SPROUT_CONSTEXPR reference ref(holder_type r) {
-				return r ? *r
-					: (throw sprout::bad_optional_access("value_holder<>: bad optional access"), *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;
diff --git a/testspr/trace.hpp b/testspr/trace.hpp
index a59e0ebe..bce04558 100644
--- a/testspr/trace.hpp
+++ b/testspr/trace.hpp
@@ -14,6 +14,7 @@
 #include <vector>
 #include <utility>
 #include <string>
+#include <sstream>
 #include <typeinfo>
 #include <iostream>
 #include <sprout/config.hpp>
@@ -58,6 +59,11 @@ namespace testspr {
 			return what_;
 		}
 	};
+	template<typename Elem, typename Traits>
+	inline SPROUT_NON_CONSTEXPR std::basic_ostream<Elem, Traits>&
+	operator<<(std::basic_ostream<Elem, Traits>& lhs, testspr::trace_info const& rhs) {
+		return lhs << rhs.file() << ':' << rhs.line() << ": " << rhs.tag() << ": " << rhs.what();
+	}
 
 	//
 	// trace_record
@@ -123,9 +129,26 @@ namespace testspr {
 			std::copy(begin(), end(), out);
 		}
 		SPROUT_NON_CONSTEXPR void print() const {
-			testspr::print_ln("trace-record current: size:", size(), ":");
+			testspr::print_ln("trace-record current: size:", size(), ':');
 			for (const_iterator it = cbegin(), last = cend(); it != last; ++it) {
-				testspr::print_ln("  ", it->first, " - ", it->second.file(), ":", it->second.line(), ": ", it->second.tag(), ": ", it->second.what());
+				testspr::print_ln("  ", it->first, " - ", it->second);
+			}
+		}
+		SPROUT_NON_CONSTEXPR void print_stratified() const {
+			testspr::print_ln("trace-record current: size:", size(), ':');
+			std::string indent;
+			for (const_iterator it = cbegin(), last = cend(); it != last; ++it) {
+				if (it->first == "push") {
+					testspr::print_ln("  ", indent, it->second, " {");
+					indent.append(2, ' ');
+				} else if (it->first == "pop") {
+					indent.erase(indent.size() - 2);
+					testspr::print_ln("  ", indent, '}');
+				} else if (it->first == "notify") {
+					testspr::print_ln("  ", indent, it->second);
+				} else {
+					testspr::print_ln("  ", indent, it->first, " - ", it->second);
+				}
 			}
 		}
 	};
@@ -175,16 +198,14 @@ namespace testspr {
 		class print_on_push {
 		public:
 			SPROUT_NON_CONSTEXPR bool operator()(testspr::trace_stack const& stack) const {
-				testspr::trace_info const& info = stack.top();
-				testspr::print_ln("trace-stack push: ", info.file(), ":", info.line(), ": ", info.tag(), ": ", info.what());
+				testspr::print_ln("trace-stack push: ", stack.top());
 				return false;
 			}
 		};
 		class print_on_pop {
 		public:
 			SPROUT_NON_CONSTEXPR bool operator()(testspr::trace_stack const& stack) const {
-				testspr::trace_info const& info = stack.top();
-				testspr::print_ln("trace-stack pop: ", info.file(), ":", info.line(), ": ", info.tag(), ": ", info.what());
+				testspr::print_ln("trace-stack pop: ", stack.top());
 				return false;
 			}
 		};
@@ -206,7 +227,7 @@ namespace testspr {
 		public:
 			SPROUT_NON_CONSTEXPR bool operator()(testspr::trace_stack const& stack) const {
 				testspr::trace_info const& info = stack.top();
-				testspr::print_ln("trace-stack notify: ", info.file(), ":", info.line(), ": ", info.tag(), ": ", info.what());
+				testspr::print_ln("trace-stack notify: ", info);
 				if (info.tag() == "assertion-failed") {
 					testspr::trace_stack::instance().print();
 					testspr::trace_record::instance().print();
@@ -304,9 +325,9 @@ namespace testspr {
 			std::copy(begin(), end(), out);
 		}
 		SPROUT_NON_CONSTEXPR void print() const {
-			testspr::print_ln("trace-stack current: size:", size(), ":");
+			testspr::print_ln("trace-stack current: size:", size(), ':');
 			for (const_iterator it = cbegin(), last = cend(); it != last; ++it) {
-				testspr::print_ln("  ", it->file(), ":", it->line(), ": ", it->tag(), ": ", it->what());
+				testspr::print_ln("  ", *it);
 			}
 		}
 
@@ -551,6 +572,12 @@ namespace testspr {
 #	define TESTSPR_TRACE_MARK(name) \
 		((void)testspr::trace_stack::instance().notify_mark(testspr::to_string(name), __FILE__, __LINE__))
 
+	//
+	// TESTSPR_PRINT_CURRENT
+	//
+#	define TESTSPR_PRINT_CURRENT(name) \
+		((void)testspr::print_ln(testspr::trace_info(__FILE__, __LINE__, "current", name)))
+
 	//
 	// TESTSPR_PRINT_TRACE_STACK
 	// TESTSPR_PRINT_TRACE_RECORD
@@ -560,6 +587,12 @@ namespace testspr {
 #	define TESTSPR_PRINT_TRACE_RECORD() \
 		((void)testspr::trace_record::instance().print())
 
+	//
+	// TESTSPR_PRINT_STRATIFIED_TRACE_RECORD
+	//
+#	define TESTSPR_PRINT_STRATIFIED_TRACE_RECORD() \
+		((void)testspr::trace_record::instance().print_stratified())
+
 
 	//
 	// TESTSPR_ENABLE_PRINT_ON_ENTRY