rename Mystery

just unknown, mystery should be more difficult to find out.
After all we keep run time info, so just need unknown at compile time

lib/virtual/type.rb

@@ -1,7 +1,7 @@
 
 module Virtual
   # Integer and (Object) References are the main derived classes, but float will come and ...
-  # The Mystery Type has unknown type and has only casting methods. So it must be cast to be useful.
+  # The Unknown Type has unknown type and has only casting methods. So it must be cast to be useful.
   class Type
     def == other
       return false unless other.class == self.class
@@ -20,7 +20,7 @@ module Virtual
     attr_reader :of_class
   end
 
-  class Mystery < Type
+  class Unknown < Type
   end
 
 end