> I have my own collection library now, developed for my own 
> education, and part of it is an implementation of Set and its 
> relatives which
> - couldn't care less about nil
> - doesn't have any other sentinel either
> - would have to waste extra time going out of its way to block nil.
> - handles filling up rather more gracefully
> (How?  See any good data structures and algorithms textbook.)
> 

Richard, do you have any plans to publish your collection library?  Say,
on SqueakMap? I'd really like to see what you've done...

Regards,
Brent