Actually Number sounds like a special case of List to me - a list with one entry. Perhaps you should do away with Number entirely, at least as a Public entity?

No. You're being excessively generic. It isn't immediately obvious what "add" means in connection with a list. My first interpretation that "add" would be applied once to each element in the list and thus return a new list, transformed. That is, map() and +. So why are you reimplementing basic things like arrays anyway? Usually, if a person wanted a sum from a list they'd say it in an obvious way like sum( @bunch_o_numbers ) (List::Util::sum) or similar. This "Value" parent class or more commonly, "Object" is a common pitfall and never, really a good idea. That is, unless you're doing something gemane to http://lambda-the-ultimate.org. Then write the paper on it and publish it for all of us to read.