diff --git a/CMakeLists.txt b/CMakeLists.txt
index 96e6c5f..b9609e7 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -73,6 +73,7 @@ add_library(athena-core
     include/athena/DNAYaml.hpp
     include/yaml.h
     include/utf8proc.h
+    include/optional.hpp
 )
 
 add_library(athena-sakura EXCLUDE_FROM_ALL
diff --git a/include/optional.hpp b/include/optional.hpp
new file mode 100644
index 0000000..6bc51df
--- /dev/null
+++ b/include/optional.hpp
@@ -0,0 +1,1045 @@
+// Copyright (C) 2011 - 2012 Andrzej Krzemienski.
+//
+// Use, modification, and distribution is subject to 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)
+//
+// The idea and interface is based on Boost.Optional library
+// authored by Fernando Luis Cacciola Carballal
+
+# ifndef ___OPTIONAL_HPP___
+# define ___OPTIONAL_HPP___
+
+# include <utility>
+# include <type_traits>
+# include <initializer_list>
+# include <cassert>
+# include <functional>
+# include <string>
+# include "athena/Global.hpp"
+
+# define TR2_OPTIONAL_REQUIRES(...) typename enable_if<__VA_ARGS__::value, bool>::type = false
+
+# if defined __GNUC__ // NOTE: GNUC is also defined for Clang
+#   if (__GNUC__ == 4) && (__GNUC_MINOR__ >= 8)
+#     define TR2_OPTIONAL_GCC_4_8_AND_HIGHER___
+#   elif (__GNUC__ > 4)
+#     define TR2_OPTIONAL_GCC_4_8_AND_HIGHER___
+#   endif
+#
+#   if (__GNUC__ == 4) && (__GNUC_MINOR__ >= 7)
+#     define TR2_OPTIONAL_GCC_4_7_AND_HIGHER___
+#   elif (__GNUC__ > 4)
+#     define TR2_OPTIONAL_GCC_4_7_AND_HIGHER___
+#   endif
+#
+#   if (__GNUC__ == 4) && (__GNUC_MINOR__ == 8) && (__GNUC_PATCHLEVEL__ >= 1)
+#     define TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___
+#   elif (__GNUC__ == 4) && (__GNUC_MINOR__ >= 9)
+#     define TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___
+#   elif (__GNUC__ > 4)
+#     define TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___
+#   endif
+# endif
+#
+# if defined __clang_major__
+#   if (__clang_major__ == 3 && __clang_minor__ >= 5)
+#     define TR2_OPTIONAL_CLANG_3_5_AND_HIGHTER_
+#   elif (__clang_major__ > 3)
+#     define TR2_OPTIONAL_CLANG_3_5_AND_HIGHTER_
+#   endif
+#   if defined TR2_OPTIONAL_CLANG_3_5_AND_HIGHTER_
+#     define TR2_OPTIONAL_CLANG_3_4_2_AND_HIGHER_
+#   elif (__clang_major__ == 3 && __clang_minor__ == 4 && __clang_patchlevel__ >= 2)
+#     define TR2_OPTIONAL_CLANG_3_4_2_AND_HIGHER_
+#   endif
+# endif
+#
+# if defined _MSC_VER
+#   if (_MSC_VER >= 1900)
+#     define TR2_OPTIONAL_MSVC_2015_AND_HIGHER___
+#   endif
+# endif
+
+# if defined __clang__
+#   if (__clang_major__ > 2) || (__clang_major__ == 2) && (__clang_minor__ >= 9)
+#     define OPTIONAL_HAS_THIS_RVALUE_REFS 1
+#   else
+#     define OPTIONAL_HAS_THIS_RVALUE_REFS 0
+#   endif
+# elif defined TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___
+#   define OPTIONAL_HAS_THIS_RVALUE_REFS 1
+# elif defined TR2_OPTIONAL_MSVC_2015_AND_HIGHER___
+#   define OPTIONAL_HAS_THIS_RVALUE_REFS 1
+# else
+#   define OPTIONAL_HAS_THIS_RVALUE_REFS 0
+# endif
+
+
+# if defined TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___
+#   define OPTIONAL_HAS_CONSTEXPR_INIT_LIST 1
+#   define OPTIONAL_CONSTEXPR_INIT_LIST constexpr
+# else
+#   define OPTIONAL_HAS_CONSTEXPR_INIT_LIST 0
+#   define OPTIONAL_CONSTEXPR_INIT_LIST
+# endif
+
+# if defined TR2_OPTIONAL_CLANG_3_5_AND_HIGHTER_ && (defined __cplusplus) && (__cplusplus != 201103L)
+#   define OPTIONAL_HAS_MOVE_ACCESSORS 1
+# else
+#   define OPTIONAL_HAS_MOVE_ACCESSORS 0
+# endif
+
+# // In C++11 constexpr implies const, so we need to make non-const members also non-constexpr
+# if (defined __cplusplus) && (__cplusplus == 201103L)
+#   define OPTIONAL_MUTABLE_CONSTEXPR
+# else
+#   define OPTIONAL_MUTABLE_CONSTEXPR constexpr
+# endif
+
+namespace std{
+
+namespace experimental{
+
+// BEGIN workaround for missing is_trivially_destructible
+# if defined TR2_OPTIONAL_GCC_4_8_AND_HIGHER___
+    // leave it: it is already there
+# elif defined TR2_OPTIONAL_CLANG_3_4_2_AND_HIGHER_
+    // leave it: it is already there
+# elif defined TR2_OPTIONAL_MSVC_2015_AND_HIGHER___
+    // leave it: it is already there
+# elif defined TR2_OPTIONAL_DISABLE_EMULATION_OF_TYPE_TRAITS
+    // leave it: the user doesn't want it
+# else
+    template <typename T>
+    using is_trivially_destructible = std::has_trivial_destructor<T>;
+# endif
+// END workaround for missing is_trivially_destructible
+
+# if (defined TR2_OPTIONAL_GCC_4_7_AND_HIGHER___)
+    // leave it; our metafunctions are already defined.
+# elif defined TR2_OPTIONAL_CLANG_3_4_2_AND_HIGHER_
+    // leave it; our metafunctions are already defined.
+# elif defined TR2_OPTIONAL_MSVC_2015_AND_HIGHER___
+    // leave it: it is already there
+# elif defined TR2_OPTIONAL_DISABLE_EMULATION_OF_TYPE_TRAITS
+    // leave it: the user doesn't want it
+# else
+
+
+// workaround for missing traits in GCC and CLANG
+template <class T>
+struct is_nothrow_move_constructible
+{
+  constexpr static bool value = std::is_nothrow_constructible<T, T&&>::value;
+};
+
+
+template <class T, class U>
+struct is_assignable
+{
+  template <class X, class Y>
+  constexpr static bool has_assign(...) { return false; }
+
+  template <class X, class Y, size_t S = sizeof((std::declval<X>() = std::declval<Y>(), true)) >
+  // the comma operator is necessary for the cases where operator= returns void
+  constexpr static bool has_assign(bool) { return true; }
+
+  constexpr static bool value = has_assign<T, U>(true);
+};
+
+
+template <class T>
+struct is_nothrow_move_assignable
+{
+  template <class X, bool has_any_move_assign>
+  struct has_nothrow_move_assign {
+    constexpr static bool value = false;
+  };
+
+  template <class X>
+  struct has_nothrow_move_assign<X, true> {
+    constexpr static bool value = noexcept( std::declval<X&>() = std::declval<X&&>() );
+  };
+
+  constexpr static bool value = has_nothrow_move_assign<T, is_assignable<T&, T&&>::value>::value;
+};
+// end workaround
+
+
+# endif
+
+
+
+// 20.5.4, optional for object types
+template <class T> class optional;
+
+// 20.5.5, optional for lvalue reference types
+template <class T> class optional<T&>;
+
+
+// workaround: std utility functions aren't constexpr yet
+template <class T> inline constexpr T&& constexpr_forward(typename std::remove_reference<T>::type& t) noexcept
+{
+  return static_cast<T&&>(t);
+}
+
+template <class T> inline constexpr T&& constexpr_forward(typename std::remove_reference<T>::type&& t) noexcept
+{
+    static_assert(!std::is_lvalue_reference<T>::value, "!!");
+    return static_cast<T&&>(t);
+}
+
+template <class T> inline constexpr typename std::remove_reference<T>::type&& constexpr_move(T&& t) noexcept
+{
+    return static_cast<typename std::remove_reference<T>::type&&>(t);
+}
+
+
+#if defined NDEBUG
+# define TR2_OPTIONAL_ASSERTED_EXPRESSION(CHECK, EXPR) (EXPR)
+#else
+# define TR2_OPTIONAL_ASSERTED_EXPRESSION(CHECK, EXPR) ((CHECK) ? (EXPR) : ([]{assert(!#CHECK);}(), (EXPR)))
+#endif
+
+
+namespace detail_
+{
+
+// static_addressof: a constexpr version of addressof
+template <typename T>
+struct has_overloaded_addressof
+{
+  template <class X>
+  constexpr static bool has_overload(...) { return false; }
+
+  template <class X, size_t S = sizeof(std::declval<X&>().operator&()) >
+  constexpr static bool has_overload(bool) { return true; }
+
+  constexpr static bool value = has_overload<T>(true);
+};
+
+template <typename T, TR2_OPTIONAL_REQUIRES(!has_overloaded_addressof<T>)>
+constexpr T* static_addressof(T& ref)
+{
+  return &ref;
+}
+
+template <typename T, TR2_OPTIONAL_REQUIRES(has_overloaded_addressof<T>)>
+T* static_addressof(T& ref)
+{
+  return std::addressof(ref);
+}
+
+
+// the call to convert<A>(b) has return type A and converts b to type A iff b decltype(b) is implicitly convertible to A
+template <class U>
+U convert(U v) { return v; }
+
+} // namespace detail
+
+
+constexpr struct trivial_init_t{} trivial_init{};
+
+
+// 20.5.6, In-place construction
+constexpr struct in_place_t{} in_place{};
+
+
+// 20.5.7, Disengaged state indicator
+struct nullopt_t
+{
+  struct init{};
+  constexpr explicit nullopt_t(init){}
+};
+constexpr nullopt_t nullopt{nullopt_t::init()};
+
+
+// 20.5.8, class bad_optional_access
+class bad_optional_access : public logic_error {
+public:
+  explicit bad_optional_access(const string& what_arg) : logic_error{what_arg} {}
+  explicit bad_optional_access(const char* what_arg) : logic_error{what_arg} {}
+};
+
+
+template <class T>
+union storage_t
+{
+  unsigned char dummy_;
+  T value_;
+
+  constexpr storage_t( trivial_init_t ) noexcept : dummy_() {};
+
+  template <class... Args>
+  constexpr storage_t( Args&&... args ) : value_(constexpr_forward<Args>(args)...) {}
+
+  ~storage_t(){}
+};
+
+
+template <class T>
+union constexpr_storage_t
+{
+    unsigned char dummy_;
+    T value_;
+
+    constexpr constexpr_storage_t( trivial_init_t ) noexcept : dummy_() {};
+
+    template <class... Args>
+    constexpr constexpr_storage_t( Args&&... args ) : value_(constexpr_forward<Args>(args)...) {}
+
+    ~constexpr_storage_t() = default;
+};
+
+
+template <class T>
+struct optional_base
+{
+    bool init_;
+    storage_t<T> storage_;
+
+    constexpr optional_base() noexcept : init_(false), storage_(trivial_init) {};
+
+    explicit constexpr optional_base(const T& v) : init_(true), storage_(v) {}
+
+    explicit constexpr optional_base(T&& v) : init_(true), storage_(constexpr_move(v)) {}
+
+    template <class... Args> explicit optional_base(in_place_t, Args&&... args)
+        : init_(true), storage_(constexpr_forward<Args>(args)...) {}
+
+    template <class U, class... Args, TR2_OPTIONAL_REQUIRES(is_constructible<T, std::initializer_list<U>>)>
+    explicit optional_base(in_place_t, std::initializer_list<U> il, Args&&... args)
+        : init_(true), storage_(il, std::forward<Args>(args)...) {}
+
+    ~optional_base() { if (init_) storage_.value_.T::~T(); }
+};
+
+
+template <class T>
+struct constexpr_optional_base
+{
+    bool init_;
+    constexpr_storage_t<T> storage_;
+
+    constexpr constexpr_optional_base() noexcept : init_(false), storage_(trivial_init) {};
+
+    explicit constexpr constexpr_optional_base(const T& v) : init_(true), storage_(v) {}
+
+    explicit constexpr constexpr_optional_base(T&& v) : init_(true), storage_(constexpr_move(v)) {}
+
+    template <class... Args> explicit constexpr constexpr_optional_base(in_place_t, Args&&... args)
+      : init_(true), storage_(constexpr_forward<Args>(args)...) {}
+
+    template <class U, class... Args, TR2_OPTIONAL_REQUIRES(is_constructible<T, std::initializer_list<U>>)>
+    OPTIONAL_CONSTEXPR_INIT_LIST explicit constexpr_optional_base(in_place_t, std::initializer_list<U> il, Args&&... args)
+      : init_(true), storage_(il, std::forward<Args>(args)...) {}
+
+    ~constexpr_optional_base() = default;
+};
+
+template <class T>
+using OptionalBase = typename std::conditional<
+    is_trivially_destructible<T>::value,
+    constexpr_optional_base<T>,
+    optional_base<T>
+>::type;
+
+
+
+template <class T>
+class optional : private OptionalBase<T>
+{
+  static_assert( !std::is_same<typename std::decay<T>::type, nullopt_t>::value, "bad T" );
+  static_assert( !std::is_same<typename std::decay<T>::type, in_place_t>::value, "bad T" );
+
+
+  constexpr bool initialized() const noexcept { return OptionalBase<T>::init_; }
+  T* dataptr() {  return std::addressof(OptionalBase<T>::storage_.value_); }
+  constexpr const T* dataptr() const { return detail_::static_addressof(OptionalBase<T>::storage_.value_); }
+
+# if OPTIONAL_HAS_THIS_RVALUE_REFS == 1
+  constexpr const T& contained_val() const& { return OptionalBase<T>::storage_.value_; }
+#   if OPTIONAL_HAS_MOVE_ACCESSORS == 1
+  OPTIONAL_MUTABLE_CONSTEXPR T&& contained_val() && { return std::move(OptionalBase<T>::storage_.value_); }
+  OPTIONAL_MUTABLE_CONSTEXPR T& contained_val() & { return OptionalBase<T>::storage_.value_; }
+#   else
+  T& contained_val() & { return OptionalBase<T>::storage_.value_; }
+  T&& contained_val() && { return std::move(OptionalBase<T>::storage_.value_); }
+#   endif
+# else
+  constexpr const T& contained_val() const { return OptionalBase<T>::storage_.value_; }
+  T& contained_val() { return OptionalBase<T>::storage_.value_; }
+# endif
+
+  void clear() noexcept {
+    if (initialized()) dataptr()->T::~T();
+    OptionalBase<T>::init_ = false;
+  }
+
+  template <class... Args>
+  void initialize(Args&&... args) noexcept(noexcept(T(std::forward<Args>(args)...)))
+  {
+    assert(!OptionalBase<T>::init_);
+    ::new (static_cast<void*>(dataptr())) T(std::forward<Args>(args)...);
+    OptionalBase<T>::init_ = true;
+  }
+
+  template <class U, class... Args>
+  void initialize(std::initializer_list<U> il, Args&&... args) noexcept(noexcept(T(il, std::forward<Args>(args)...)))
+  {
+    assert(!OptionalBase<T>::init_);
+    ::new (static_cast<void*>(dataptr())) T(il, std::forward<Args>(args)...);
+    OptionalBase<T>::init_ = true;
+  }
+
+public:
+  typedef T value_type;
+
+  // 20.5.5.1, constructors
+  constexpr optional() noexcept : OptionalBase<T>()  {};
+  constexpr optional(nullopt_t) noexcept : OptionalBase<T>() {};
+
+  optional(const optional& rhs)
+  : OptionalBase<T>()
+  {
+    if (rhs.initialized()) {
+        ::new (static_cast<void*>(dataptr())) T(*rhs);
+        OptionalBase<T>::init_ = true;
+    }
+  }
+
+  optional(optional&& rhs) noexcept(is_nothrow_move_constructible<T>::value)
+  : OptionalBase<T>()
+  {
+    if (rhs.initialized()) {
+        ::new (static_cast<void*>(dataptr())) T(std::move(*rhs));
+        OptionalBase<T>::init_ = true;
+    }
+  }
+
+  constexpr optional(const T& v) : OptionalBase<T>(v) {}
+
+  constexpr optional(T&& v) : OptionalBase<T>(constexpr_move(v)) {}
+
+  template <class... Args>
+  explicit constexpr optional(in_place_t, Args&&... args)
+  : OptionalBase<T>(in_place_t{}, constexpr_forward<Args>(args)...) {}
+
+  template <class U, class... Args, TR2_OPTIONAL_REQUIRES(is_constructible<T, std::initializer_list<U>>)>
+  OPTIONAL_CONSTEXPR_INIT_LIST explicit optional(in_place_t, std::initializer_list<U> il, Args&&... args)
+  : OptionalBase<T>(in_place_t{}, il, constexpr_forward<Args>(args)...) {}
+
+  // 20.5.4.2, Destructor
+  ~optional() = default;
+
+  // 20.5.4.3, assignment
+  optional& operator=(nullopt_t) noexcept
+  {
+    clear();
+    return *this;
+  }
+
+  optional& operator=(const optional& rhs)
+  {
+    if      (initialized() == true  && rhs.initialized() == false) clear();
+    else if (initialized() == false && rhs.initialized() == true)  initialize(*rhs);
+    else if (initialized() == true  && rhs.initialized() == true)  contained_val() = *rhs;
+    return *this;
+  }
+
+  optional& operator=(optional&& rhs)
+  noexcept(is_nothrow_move_assignable<T>::value && is_nothrow_move_constructible<T>::value)
+  {
+    if      (initialized() == true  && rhs.initialized() == false) clear();
+    else if (initialized() == false && rhs.initialized() == true)  initialize(std::move(*rhs));
+    else if (initialized() == true  && rhs.initialized() == true)  contained_val() = std::move(*rhs);
+    return *this;
+  }
+
+  template <class U>
+  auto operator=(U&& v)
+  -> typename enable_if
+  <
+    is_same<typename decay<U>::type, T>::value,
+    optional&
+  >::type
+  {
+    if (initialized()) { contained_val() = std::forward<U>(v); }
+    else               { initialize(std::forward<U>(v));  }
+    return *this;
+  }
+
+
+  template <class... Args>
+  void emplace(Args&&... args)
+  {
+    clear();
+    initialize(std::forward<Args>(args)...);
+  }
+
+  template <class U, class... Args>
+  void emplace(initializer_list<U> il, Args&&... args)
+  {
+    clear();
+    initialize<U, Args...>(il, std::forward<Args>(args)...);
+  }
+
+  // 20.5.4.4, Swap
+  void swap(optional<T>& rhs) noexcept(is_nothrow_move_constructible<T>::value && noexcept(swap(declval<T&>(), declval<T&>())))
+  {
+    if      (initialized() == true  && rhs.initialized() == false) { rhs.initialize(std::move(**this)); clear(); }
+    else if (initialized() == false && rhs.initialized() == true)  { initialize(std::move(*rhs)); rhs.clear(); }
+    else if (initialized() == true  && rhs.initialized() == true)  { using std::swap; swap(**this, *rhs); }
+  }
+
+  // 20.5.4.5, Observers
+
+  explicit constexpr operator bool() const noexcept { return initialized(); }
+
+  constexpr T const* operator ->() const {
+    return TR2_OPTIONAL_ASSERTED_EXPRESSION(initialized(), dataptr());
+  }
+
+# if OPTIONAL_HAS_MOVE_ACCESSORS == 1
+
+  OPTIONAL_MUTABLE_CONSTEXPR T* operator ->() {
+    assert (initialized());
+    return dataptr();
+  }
+
+  constexpr T const& operator *() const& {
+    return TR2_OPTIONAL_ASSERTED_EXPRESSION(initialized(), contained_val());
+  }
+
+  OPTIONAL_MUTABLE_CONSTEXPR T& operator *() & {
+    assert (initialized());
+    return contained_val();
+  }
+
+  OPTIONAL_MUTABLE_CONSTEXPR T&& operator *() && {
+    assert (initialized());
+    return constexpr_move(contained_val());
+  }
+
+  constexpr T const& value() const& {
+    if (!initialized()) atFatal("bad optional access");
+    return contained_val();
+  }
+
+  OPTIONAL_MUTABLE_CONSTEXPR T& value() & {
+    if (!initialized()) atFatal("bad optional access");
+    return contained_val();
+  }
+
+  OPTIONAL_MUTABLE_CONSTEXPR T&& value() && {
+    if (!initialized()) atFatal("bad optional access");
+    return std::move(contained_val());
+  }
+
+# else
+
+  T* operator ->() {
+    assert (initialized());
+    return dataptr();
+  }
+
+  constexpr T const& operator *() const {
+    return TR2_OPTIONAL_ASSERTED_EXPRESSION(initialized(), contained_val());
+  }
+
+  T& operator *() {
+    assert (initialized());
+    return contained_val();
+  }
+
+  constexpr T const& value() const {
+    return initialized() ? contained_val() : atFatal("bad optional access");
+  }
+
+  T& value() {
+    return initialized() ? contained_val() : atFatal("bad optional access");
+  }
+
+# endif
+
+# if OPTIONAL_HAS_THIS_RVALUE_REFS == 1
+
+  template <class V>
+  constexpr T value_or(V&& v) const&
+  {
+    return *this ? **this : detail_::convert<T>(constexpr_forward<V>(v));
+  }
+
+#   if OPTIONAL_HAS_MOVE_ACCESSORS == 1
+
+  template <class V>
+  OPTIONAL_MUTABLE_CONSTEXPR T value_or(V&& v) &&
+  {
+    return *this ? constexpr_move(const_cast<optional<T>&>(*this).contained_val()) : detail_::convert<T>(constexpr_forward<V>(v));
+  }
+
+#   else
+
+  template <class V>
+  T value_or(V&& v) &&
+  {
+    return *this ? constexpr_move(const_cast<optional<T>&>(*this).contained_val()) : detail_::convert<T>(constexpr_forward<V>(v));
+  }
+
+#   endif
+
+# else
+
+  template <class V>
+  constexpr T value_or(V&& v) const
+  {
+    return *this ? **this : detail_::convert<T>(constexpr_forward<V>(v));
+  }
+
+# endif
+
+};
+
+
+template <class T>
+class optional<T&>
+{
+  static_assert( !std::is_same<T, nullopt_t>::value, "bad T" );
+  static_assert( !std::is_same<T, in_place_t>::value, "bad T" );
+  T* ref;
+
+public:
+
+  // 20.5.5.1, construction/destruction
+  constexpr optional() noexcept : ref(nullptr) {}
+
+  constexpr optional(nullopt_t) noexcept : ref(nullptr) {}
+
+  constexpr optional(T& v) noexcept : ref(detail_::static_addressof(v)) {}
+
+  optional(T&&) = delete;
+
+  constexpr optional(const optional& rhs) noexcept : ref(rhs.ref) {}
+
+  explicit constexpr optional(in_place_t, T& v) noexcept : ref(detail_::static_addressof(v)) {}
+
+  explicit optional(in_place_t, T&&) = delete;
+
+  ~optional() = default;
+
+  // 20.5.5.2, mutation
+  optional& operator=(nullopt_t) noexcept {
+    ref = nullptr;
+    return *this;
+  }
+
+  // optional& operator=(const optional& rhs) noexcept {
+    // ref = rhs.ref;
+    // return *this;
+  // }
+
+  // optional& operator=(optional&& rhs) noexcept {
+    // ref = rhs.ref;
+    // return *this;
+  // }
+
+  template <typename U>
+  auto operator=(U&& rhs) noexcept
+  -> typename enable_if
+  <
+    is_same<typename decay<U>::type, optional<T&>>::value,
+    optional&
+  >::type
+  {
+    ref = rhs.ref;
+    return *this;
+  }
+
+  template <typename U>
+  auto operator=(U&& rhs) noexcept
+  -> typename enable_if
+  <
+    !is_same<typename decay<U>::type, optional<T&>>::value,
+    optional&
+  >::type
+  = delete;
+
+  void emplace(T& v) noexcept {
+    ref = detail_::static_addressof(v);
+  }
+
+  void emplace(T&&) = delete;
+
+
+  void swap(optional<T&>& rhs) noexcept
+  {
+    std::swap(ref, rhs.ref);
+  }
+
+  // 20.5.5.3, observers
+  constexpr T* operator->() const {
+    return TR2_OPTIONAL_ASSERTED_EXPRESSION(ref, ref);
+  }
+
+  constexpr T& operator*() const {
+    return TR2_OPTIONAL_ASSERTED_EXPRESSION(ref, *ref);
+  }
+
+  constexpr T& value() const {
+    if (!ref) atFatal("bad optional access");
+    return *ref;
+  }
+
+  explicit constexpr operator bool() const noexcept {
+    return ref != nullptr;
+  }
+
+  template <class V>
+  constexpr typename decay<T>::type value_or(V&& v) const
+  {
+    return *this ? **this : detail_::convert<typename decay<T>::type>(constexpr_forward<V>(v));
+  }
+};
+
+
+template <class T>
+class optional<T&&>
+{
+  static_assert( sizeof(T) == 0, "optional rvalue references disallowed" );
+};
+
+
+// 20.5.8, Relational operators
+template <class T> constexpr bool operator==(const optional<T>& x, const optional<T>& y)
+{
+  return bool(x) != bool(y) ? false : bool(x) == false ? true : *x == *y;
+}
+
+template <class T> constexpr bool operator!=(const optional<T>& x, const optional<T>& y)
+{
+  return !(x == y);
+}
+
+template <class T> constexpr bool operator<(const optional<T>& x, const optional<T>& y)
+{
+  return (!y) ? false : (!x) ? true : *x < *y;
+}
+
+template <class T> constexpr bool operator>(const optional<T>& x, const optional<T>& y)
+{
+  return (y < x);
+}
+
+template <class T> constexpr bool operator<=(const optional<T>& x, const optional<T>& y)
+{
+  return !(y < x);
+}
+
+template <class T> constexpr bool operator>=(const optional<T>& x, const optional<T>& y)
+{
+  return !(x < y);
+}
+
+
+// 20.5.9, Comparison with nullopt
+template <class T> constexpr bool operator==(const optional<T>& x, nullopt_t) noexcept
+{
+  return (!x);
+}
+
+template <class T> constexpr bool operator==(nullopt_t, const optional<T>& x) noexcept
+{
+  return (!x);
+}
+
+template <class T> constexpr bool operator!=(const optional<T>& x, nullopt_t) noexcept
+{
+  return bool(x);
+}
+
+template <class T> constexpr bool operator!=(nullopt_t, const optional<T>& x) noexcept
+{
+  return bool(x);
+}
+
+template <class T> constexpr bool operator<(const optional<T>&, nullopt_t) noexcept
+{
+  return false;
+}
+
+template <class T> constexpr bool operator<(nullopt_t, const optional<T>& x) noexcept
+{
+  return bool(x);
+}
+
+template <class T> constexpr bool operator<=(const optional<T>& x, nullopt_t) noexcept
+{
+  return (!x);
+}
+
+template <class T> constexpr bool operator<=(nullopt_t, const optional<T>&) noexcept
+{
+  return true;
+}
+
+template <class T> constexpr bool operator>(const optional<T>& x, nullopt_t) noexcept
+{
+  return bool(x);
+}
+
+template <class T> constexpr bool operator>(nullopt_t, const optional<T>&) noexcept
+{
+  return false;
+}
+
+template <class T> constexpr bool operator>=(const optional<T>&, nullopt_t) noexcept
+{
+  return true;
+}
+
+template <class T> constexpr bool operator>=(nullopt_t, const optional<T>& x) noexcept
+{
+  return (!x);
+}
+
+
+
+// 20.5.10, Comparison with T
+template <class T> constexpr bool operator==(const optional<T>& x, const T& v)
+{
+  return bool(x) ? *x == v : false;
+}
+
+template <class T> constexpr bool operator==(const T& v, const optional<T>& x)
+{
+  return bool(x) ? v == *x : false;
+}
+
+template <class T> constexpr bool operator!=(const optional<T>& x, const T& v)
+{
+  return bool(x) ? *x != v : true;
+}
+
+template <class T> constexpr bool operator!=(const T& v, const optional<T>& x)
+{
+  return bool(x) ? v != *x : true;
+}
+
+template <class T> constexpr bool operator<(const optional<T>& x, const T& v)
+{
+  return bool(x) ? *x < v : true;
+}
+
+template <class T> constexpr bool operator>(const T& v, const optional<T>& x)
+{
+  return bool(x) ? v > *x : true;
+}
+
+template <class T> constexpr bool operator>(const optional<T>& x, const T& v)
+{
+  return bool(x) ? *x > v : false;
+}
+
+template <class T> constexpr bool operator<(const T& v, const optional<T>& x)
+{
+  return bool(x) ? v < *x : false;
+}
+
+template <class T> constexpr bool operator>=(const optional<T>& x, const T& v)
+{
+  return bool(x) ? *x >= v : false;
+}
+
+template <class T> constexpr bool operator<=(const T& v, const optional<T>& x)
+{
+  return bool(x) ? v <= *x : false;
+}
+
+template <class T> constexpr bool operator<=(const optional<T>& x, const T& v)
+{
+  return bool(x) ? *x <= v : true;
+}
+
+template <class T> constexpr bool operator>=(const T& v, const optional<T>& x)
+{
+  return bool(x) ? v >= *x : true;
+}
+
+
+// Comparison of optional<T&> with T
+template <class T> constexpr bool operator==(const optional<T&>& x, const T& v)
+{
+  return bool(x) ? *x == v : false;
+}
+
+template <class T> constexpr bool operator==(const T& v, const optional<T&>& x)
+{
+  return bool(x) ? v == *x : false;
+}
+
+template <class T> constexpr bool operator!=(const optional<T&>& x, const T& v)
+{
+  return bool(x) ? *x != v : true;
+}
+
+template <class T> constexpr bool operator!=(const T& v, const optional<T&>& x)
+{
+  return bool(x) ? v != *x : true;
+}
+
+template <class T> constexpr bool operator<(const optional<T&>& x, const T& v)
+{
+  return bool(x) ? *x < v : true;
+}
+
+template <class T> constexpr bool operator>(const T& v, const optional<T&>& x)
+{
+  return bool(x) ? v > *x : true;
+}
+
+template <class T> constexpr bool operator>(const optional<T&>& x, const T& v)
+{
+  return bool(x) ? *x > v : false;
+}
+
+template <class T> constexpr bool operator<(const T& v, const optional<T&>& x)
+{
+  return bool(x) ? v < *x : false;
+}
+
+template <class T> constexpr bool operator>=(const optional<T&>& x, const T& v)
+{
+  return bool(x) ? *x >= v : false;
+}
+
+template <class T> constexpr bool operator<=(const T& v, const optional<T&>& x)
+{
+  return bool(x) ? v <= *x : false;
+}
+
+template <class T> constexpr bool operator<=(const optional<T&>& x, const T& v)
+{
+  return bool(x) ? *x <= v : true;
+}
+
+template <class T> constexpr bool operator>=(const T& v, const optional<T&>& x)
+{
+  return bool(x) ? v >= *x : true;
+}
+
+// Comparison of optional<T const&> with T
+template <class T> constexpr bool operator==(const optional<const T&>& x, const T& v)
+{
+  return bool(x) ? *x == v : false;
+}
+
+template <class T> constexpr bool operator==(const T& v, const optional<const T&>& x)
+{
+  return bool(x) ? v == *x : false;
+}
+
+template <class T> constexpr bool operator!=(const optional<const T&>& x, const T& v)
+{
+  return bool(x) ? *x != v : true;
+}
+
+template <class T> constexpr bool operator!=(const T& v, const optional<const T&>& x)
+{
+  return bool(x) ? v != *x : true;
+}
+
+template <class T> constexpr bool operator<(const optional<const T&>& x, const T& v)
+{
+  return bool(x) ? *x < v : true;
+}
+
+template <class T> constexpr bool operator>(const T& v, const optional<const T&>& x)
+{
+  return bool(x) ? v > *x : true;
+}
+
+template <class T> constexpr bool operator>(const optional<const T&>& x, const T& v)
+{
+  return bool(x) ? *x > v : false;
+}
+
+template <class T> constexpr bool operator<(const T& v, const optional<const T&>& x)
+{
+  return bool(x) ? v < *x : false;
+}
+
+template <class T> constexpr bool operator>=(const optional<const T&>& x, const T& v)
+{
+  return bool(x) ? *x >= v : false;
+}
+
+template <class T> constexpr bool operator<=(const T& v, const optional<const T&>& x)
+{
+  return bool(x) ? v <= *x : false;
+}
+
+template <class T> constexpr bool operator<=(const optional<const T&>& x, const T& v)
+{
+  return bool(x) ? *x <= v : true;
+}
+
+template <class T> constexpr bool operator>=(const T& v, const optional<const T&>& x)
+{
+  return bool(x) ? v >= *x : true;
+}
+
+
+// 20.5.12, Specialized algorithms
+template <class T>
+void swap(optional<T>& x, optional<T>& y) noexcept(noexcept(x.swap(y)))
+{
+  x.swap(y);
+}
+
+
+template <class T>
+constexpr optional<typename decay<T>::type> make_optional(T&& v)
+{
+  return optional<typename decay<T>::type>(constexpr_forward<T>(v));
+}
+
+template <class X>
+constexpr optional<X&> make_optional(reference_wrapper<X> v)
+{
+  return optional<X&>(v.get());
+}
+
+
+} // namespace experimental
+} // namespace std
+
+namespace std
+{
+  template <typename T>
+  struct hash<std::experimental::optional<T>>
+  {
+    typedef typename hash<T>::result_type result_type;
+    typedef std::experimental::optional<T> argument_type;
+
+    constexpr result_type operator()(argument_type const& arg) const {
+      return arg ? std::hash<T>{}(*arg) : result_type{};
+    }
+  };
+
+  template <typename T>
+  struct hash<std::experimental::optional<T&>>
+  {
+    typedef typename hash<T>::result_type result_type;
+    typedef std::experimental::optional<T&> argument_type;
+
+    constexpr result_type operator()(argument_type const& arg) const {
+      return arg ? std::hash<T>{}(*arg) : result_type{};
+    }
+  };
+}
+
+# undef TR2_OPTIONAL_REQUIRES
+# undef TR2_OPTIONAL_ASSERTED_EXPRESSION
+
+# endif //___OPTIONAL_HPP___