diff --git a/.gitignore b/.gitignore
index f30e0fb..a23e8d9 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,4 @@
 obj/
-lib*.so
+lib*.so*
 lib*.a
 *.o
diff --git a/include/either.hh b/include/either.hh
index 3c7979b..3e7a290 100644
--- a/include/either.hh
+++ b/include/either.hh
@@ -4,13 +4,16 @@ namespace exopt::types {
 	namespace either [[gnu::visibility("internal")]] {
 		//TODO: A version with std::shared_ptr<T> instead
 		template<typename T>//, typename P = T*>
-		class Cow final {
+		class Cow {
 			using var_t =	std::variant<pointer_type  // Borrowed
 					, value_type>;  // Owned
 		public:
 			//TODO: Use pointer traits of P to deduce `pointer_type`, so we can have P be a shared_ptr too. Or perhaps, a specialisation for `Cow<std::shared_ptr<T>>` would be better
 			using value_type = std::remove_cvref_t<T>;
 
+			constexpr static inline bool Writeable = std::is_copy_constructible_v<value_type>;
+			constexpr static inline bool NTWriteable = std::is_nothrow_copy_constructible_v<value_type>;
+
 			using reference_type = value_type &;
 			using const_reference_type = value_type const&;
 			using move_reference_type = value_type &&;
@@ -38,7 +41,6 @@ namespace exopt::types {
 					return { *pref };
 				else return { nullptr };
 			}
-			//TODO: into_owned(){, const}, to_owned(){, const}{&, &&}, etc..
 			//TODO: Operator overloads for access to `value_type`: operator*, operator->, etc.
 
 			constexpr Cow(Cow&& move) noexcept(std::is_nothrow_move_constructible_v<value_type>) {
@@ -59,46 +61,82 @@ namespace exopt::types {
 			constexpr ~Cow() noexcept(std::is_nothrow_destructible_v<value_type>) = default;
 
 			constexpr move_reference_type as_ref() && {
-				std::visit([](auto&& arg) -> move_reference_type {
-					using U = std::remove_cvref_t<decltype(arg)>;
-					if constexpr(std::is_same_v<U, value_type)
-						return std::move(arg);
-					else 	return std::move(*arg);
-				}, m_value);
+				return make_owned();
 			}
 			constexpr reference_type as_ref() & {
-				std::visit([](auto& arg) -> reference_type {
-					using U = std::remove_cvref_t<decltype(arg)>;
-					if constexpr(std::is_same_v<U, value_type)
-						return std::forward<decltype(arg)>(arg);
-					else 	return *arg;
-				}, m_value);
+				return make_owned();
 			}
 			constexpr const_reference_type as_ref() const {
-				std::visit([](const auto& arg) -> const_reference_type {
-					using U = std::remove_cvref_t<decltype(arg)>;
+				return std::visit([](const auto& arg) -> const_reference_type {
+					using U = std::remove_cvref_t<decltype(arg)>>;
 					if constexpr(std::is_same_v<U, value_type)
 						return std::forward<decltype(arg)>(arg);
 					else 	return *arg;
 				}, m_value);
 			}
-			//TODO: ^ is_empty() null-checks for `*arg`.
+			//TODO: ^ UNLIKELY(is_empty()) null-checks for `*arg`.
 
 			constexpr const_pointer_type get() const noexcept { 
-				std::visit([](const auto& arg) -> const_pointer_type {
+				return std::visit([](const auto& arg) -> const_pointer_type {
 					using U = std::remove_cvref_t<decltype(arg)>;
-					if constexpr(std::is_same_v<U, value_type)
+					if constexpr(std::is_same_v<U, value_type>)
 						return std::addressof(arg);
 					else 	return arg;
 				}, m_value);
 			}
 			constexpr pointer_type get() noexcept { 
-				std::visit([](auto& arg) -> pointer_type {
+				return std::addressof(make_owned());
+				/*
+				return std::visit([](auto& arg) -> pointer_type {
 					using U = std::remove_cvref_t<decltype(arg)>;
-					if constexpr(std::is_same_v<U, value_type)
+					if constexpr(std::is_same_v<U, value_type>)
 						return std::addressof(arg);
 					else 	return arg;
-				}, m_value);
+				}, m_value);*/
+			}
+			constexpr reference_type make_owned() & noexcept(NTWriteable) requires(Writeable) {
+				if(const auto* ct_from = std::visit([&m_value](auto& arg) -> auto* {
+					using U = std::remove_cvref_t<decltype(arg)>;
+
+					if constexpr(std::is_same_v<U, pointer_type>) return arg;
+					else return nullptr;
+					//if constexpr(std::is_same_v<U, value_type>) return false;
+					//else if(auto * refp = std::get_if<pointer_type>(m_value))
+
+				}, m_value)) return m_value.emplace<value_type>(*ct_from); //XXX: Do we need to create a temporary here for creating invariants by reading the old invariant?
+			}
+			constexpr move_reference_type make_owned() && noexcept(NTWriteable) requires(Writeable) {
+				if(const auto* ct_from = std::visit([&m_value](auto&& arg) -> auto* {
+					using U = std::remove_cvref_t<decltype(arg)>;
+
+					//TODO: I don't think this works the way I want it to... We'll need to emplace inside this function I think, since the moved temporary has been moved. It might not matter though.
+					if constexpr(std::is_same_v<U, pointer_type>) return arg;
+					else return nullptr;
+					//if constexpr(std::is_same_v<U, value_type>) return false;
+					//else if(auto * refp = std::get_if<pointer_type>(m_value))
+
+				}, std::move(m_value))) return std::move(m_value.emplace<value_type>(*ct_from)); //XXX: Do we need to create a temporary here for creating invariants by reading the old invariant?
+			}
+
+
+			//These commented out visits modify the pointee of the borrowed invariant. THIS IS NOT WHAT WE WANT. Instead, we emplace a copy and then copy it again (possibly elided: &) or return it as an rvalue reference (elided: &&)
+			constexpr value_type into_owned() & noexcept(NTWriteable) requires(Writeable) {
+				return make_owned();
+				/*return std::visit([](auto&& arg) -> move_reference_type {
+					using U = std::remove_cvref_t<decltype(arg)>;
+					if constexpr(std::is_same_v<U, value_type>)
+						return std::move(arg);
+					else 	return std::move(*arg);
+				}, std::move(m_value));*/
+			}
+			constexpr move_reference_type into_owned() && noexcept(NTWriteable) requires(Writeable) {
+				return make_owned();
+				/*return std::move(std::visit([](auto&& arg) -> move_reference_type {
+					using U = std::remove_cvref_t<decltype(arg)>;
+					if constexpr(std::is_same_v<U, value_type>)
+						return std::move(arg);
+					else 	return std::move(*arg);
+				}, std::move(m_value)));*/
 			}
 		protected:
 			constexpr bool is_empty() const noexcept {
@@ -114,7 +152,7 @@ namespace exopt::types {
 		};
 
 		template<typename T>
-		class Cow<std::shared_ptr<T>> final {
+		class Cow<std::shared_ptr<T>> {
 			using var_t = std::variant<
 					std::weak_ptr<value_type> // Borrowed
 					, std::shared_ptr<value_type>>; // Owned
diff --git a/include/pointer.h b/include/pointer.h
index 7f4eaaf..992b296 100644
--- a/include/pointer.h
+++ b/include/pointer.h
@@ -1,5 +1,7 @@
 #pragma once
 
+#include <utility>
+#include <functional>
 #include <concepts>
 #include <bit>
 #include <stdexcept>
@@ -138,6 +140,12 @@ namespace exopt::ptr {
 
 	template<typename T> requires(!std::is_reference_v<T>)
 	struct NonNull final {
+
+		using pointer_type = T*;
+		using const_pointer_type = std::remove_cv_t<T> const*;
+
+		constexpr static inline bool is_mutable = !std::is_same_v<pointer_type, const_pointer_type>;
+
 		constexpr NonNull(T& ref) noexcept
 			: ptr_(std::addressof(ref)) {}
 		[[gnu::nonnull(1, 2)]]
@@ -150,15 +158,7 @@ namespace exopt::ptr {
 		constexpr ~NonNull() noexcept = default;
 
 		constexpr static NonNull<T> try_new(T* ptr) {
-			constexpr auto _throw = [] [[noreturn, gnu::noinline, gnu::cold]] () {
-				if consteval {
-					throw NullException{};
-				} else {
-					throw std::runtime_error(NullException{}.message());
-				}
-				//throw error::as_runtime_error(error::comptime_fail<NullException>());
-			};
-			if(!ptr) _throw();
+			if(!ptr) _throw_null();
 			return NonNull<T>{ptr};
 		}
 #ifdef DEBUG
@@ -176,16 +176,51 @@ namespace exopt::ptr {
 		constexpr friend bool operator!=(std::nullptr_t, const NonNull&) noexcept { return true; }
 		constexpr friend bool operator!=(const NonNull&, std::nullptr_t) noexcept { return true; }
 
-		constexpr friend auto operator<=>(const NonNull& a, const NonNull& b) noexcept { return a.ptr_ <=> b.ptr_; }
-		constexpr friend auto operator<=>(const NonNull& a, T* b) noexcept { return (!b) ? (true<=>false) : a.ptr_ <=> b; }
-		constexpr friend auto operator<=>(T* a, const NonNull& b) noexcept { return (!a) ? (false<=>true) : (a <=> b.ptr_); }
+		constexpr friend auto operator<=>(const NonNull& a, const NonNull& b) noexcept { return a.get() <=> b.get(); }
+		constexpr friend auto operator<=>(const NonNull& a, T* b) noexcept { return (!b) ? (true<=>false) : (a.get() <=> b); }
+		constexpr friend auto operator<=>(T* a, const NonNull& b) noexcept { return (!a) ? (false<=>true) : (a <=> b.get()); }
+
+		template<typename U>
+		constexpr NonNull<U> cast() const noexcept requires(requires(T* ptr) { static_cast<U*>(ptr); })
+		{
+			return NonNull<U>::new_unchecked(static_cast<U*>(get()));
+		}
+		template<typename U>
+		constexpr NonNull<U> try_cast() const requires(requires(T* ptr) { dynamic_cast<U*>(ptr); })
+		{
+			return NonNull<U>::try_new(dynamic_cast<U*>(get()));
+		}
+
+		template<typename Func> requires(std::is_invocable_v<Func, T&>)
+		constexpr auto map_value(Func const& mapper) & noexcept(std::is_nothrow_invocable_v<Func, T&>)
+			-> NonNull<std::add_pointer_t<std::invoke_result_t<Func, T&>>> //TODO: Can we extend this for: void returns, pointer (not reference) returns, maybe even std::convertible_to<?*> returns?
+		{ return { std::addressof(std::invoke(std::forward<decltype(mapper)>(mapper), static_cast<T &>(*get()))) }; }
+
+		template<typename Func> requires(std::is_invocable_v<Func, T const&>)
+		constexpr auto map_value(Func const& mapper) const& noexcept(std::is_nothrow_invocable_v<Func, T const&>)
+			-> NonNull<std::add_pointer_t<std::invoke_result_t<Func, T const&>>> //TODO: Can we extend this for: void returns, pointer (not reference) returns, maybe even std::convertible_to<?*> returns?
+		{ return { std::addressof(std::invoke(std::forward<decltype(mapper)>(mapper), static_cast<T const&>(*get()))) }; }
+	
+		template<typename Func> requires(std::is_invocable_v<Func, T&&>)
+		constexpr auto map_value(Func&& mapper) && noexcept(std::is_nothrow_invocable_v<Func, T&&>)
+			-> NonNull<std::add_pointer_t<std::invoke_result_t<Func, T&&>>> //TODO: Can we extend this for: void returns, pointer (not reference) returns, maybe even std::convertible_to<?*> returns?
+		{ return { std::addressof(std::invoke(std::forward<decltype(mapper)>(mapper), std::move(*get()))) }; }
+
+		template<typename Func> requires(std::is_invocable_v<Func, T const&&>)
+		constexpr auto map_value(Func&& mapper) const&& noexcept(std::is_nothrow_invocable_v<Func, T const&&>)
+			-> NonNull<std::add_pointer_t<std::invoke_result_t<Func, T const&&>>> //TODO: Can we extend this for: void returns, pointer (not reference) returns, maybe even std::convertible_to<?*> returns?
+		{ return { std::addressof(std::invoke(std::forward<decltype(mapper)>(mapper), std::move(static_cast<T const&>(*get())))) }; }
+
+
+		template<typename U>	
+		constexpr explicit operator NonNull<U>() const noexcept requires(requires(T* ptr) { static_cast<U*>(ptr); })
+		{
+			return cast<U>();
+		}
 
 		[[gnu::returns_nonnull]]
 		constexpr T* get() const noexcept { return ptr_; }
-/*
-		[[gnu::returns_nonnull]]
-		constexpr T* const&& get() const&& noexcept { return std::move(ptr_); }
-*/
+
 		[[gnu::returns_nonnull]]
 		constexpr operator T*() const noexcept { return ptr_; }
 
@@ -199,6 +234,15 @@ namespace exopt::ptr {
 		[[gnu::returns_nonnull]]
 		constexpr const T* operator->() const noexcept { return ptr_; }
 	private:
+		[[noreturn, gnu::noinline, gnu::cold]]
+		constexpr void _throw_null() {
+			if consteval {
+				throw NullException{};
+			} else {
+				throw std::runtime_error(NullException{}.message());
+			}
+			//throw error::as_runtime_error(error::comptime_fail<NullException>());
+		}
 		T* ptr_;
 	};
 }