types/tagged: add type query

types/tagged.hpp

@@ -82,6 +82,15 @@ namespace cruft {
         auto tag (void)       { return m_tag; }
 
 
+        /// Returns true if the contained type is the same as the type
+        /// specified in the template parameter.
+        template <typename QueryT>
+        bool is (void) const noexcept
+        {
+            return QueryT::tag == m_tag;
+        }
+
+
         ///--------------------------------------------------------------------
         /// Return a reference to a stored value of known type.
         ///