missing virtual destructor corrected

asgn/fmwkng.hpp

@@ -40,7 +40,7 @@ namespace fmwkng {
 
 		class abstract_element : immovable {
 		public:
-			~abstract_element() noexcept = default;
+			virtual ~abstract_element() noexcept = default;
 			virtual element_ptr clone() const = 0;
 			virtual std::size_t data_size() const = 0;
 			virtual void data_text(std::ostream& os, std::size_t i) const = 0;