> ....without having to create a new class for bar and define the [] 
> method in that class?

I've been puzzled by this myself. There has been many times where I've 
wanted to fake the appearance of a collection within a class without 
actually making another class just for the purpose.

Could this perhaps make it in as an RCR?
--
David Heinemeier Hansson.
http://www.loudthinking.com/ -- Broadcasting Brain