I guess you could do this in a higher-level language like Ruby, though you either have to use a mixin or monkey-patch a core class, which isn't as clean cut as

I guess you could do this in a higher-level language like Ruby, though you either have to use a mixin or monkey-patch a core class, which isn't as clean cut as a first-class construct. With Lisp, you could just create the construct if it isn't there already. No such luck in Java et al.