Design cleanup

- unknown_unit added
- examples refactored
- base_type renamed to downcast_base_type
- scaled_unit renamed to named_scaled_unit
- detail::reference_unit renamed to scaled_unit
- quantity_test cleanup

src/include/units/bits/downcasting.h

@@ -29,16 +29,16 @@ namespace units {
 
   template<typename BaseType>
   struct downcast_base {
-    using base_type = BaseType;
+    using downcast_base_type = BaseType;
     friend auto downcast_guide(downcast_base);
   };
 
   template<typename T>
   concept Downcastable =
       requires {
-        typename T::base_type;
+        typename T::downcast_base_type;
       } &&
-      std::derived_from<T, downcast_base<typename T::base_type>>;
+      std::derived_from<T, downcast_base<typename T::downcast_base_type>>;
 
   template<typename Target, Downcastable T>
   struct downcast_child : T {
@@ -67,6 +67,6 @@ namespace units {
   using downcast = decltype(detail::downcast_impl<T>());
 
   template<Downcastable T>
-  using downcast_base_t = T::base_type;
+  using downcast_base_t = T::downcast_base_type;
 
 }  // namespace units

src/include/units/bits/unit_concept.h

@@ -27,15 +27,14 @@
 
 namespace units {
 
-namespace detail {
+template<typename R>
+concept UnitRatio = Ratio<R> && (R::num * R::den > 0);
 
-template<typename U, Ratio R>
-struct reference_unit;
-
-}  // namespace detail
+template<typename U, UnitRatio R>
+struct scaled_unit;
 
 // Unit
 template<typename T>
-concept Unit = is_derived_from_instantiation<T, detail::reference_unit>;
+concept Unit = is_derived_from_instantiation<T, scaled_unit>;
 
 }  // namespace units