[Grace-core] Typing of Number

[James Noble]

> The idea is that as well as self-type annotations and bounded type
> variables there would be the possibility of referring to the
> least-upper-bound type of two types,

So I was looking at the website for Ceylon

http://in.relation.to/Bloggers/IntroductionToCeylonPart10

and it seems they have a way of doing this without self-types
or LUB - but rather using annotations and union types.
(e.g. an interface Castable<> which acts as an annotation
describing to what types an object can be cast).

How do you this approach compares?

James