Levente Uzonyi wrote:
> > Which message send would you save in
> >
> > 	... == UnoccupiedSlot
> >
> 
> UnoccupiedSlot instance.
> I wouldn't use a class, because I think we should be able create a set 
> with any classes in it.

Indeed - I am sure there are several places in the image that deal with
sets of all classes or all globals.

One example of a Smalltalk with a non nil object to mark empty slots in
sets is Self, so it has been done before. On the other hand, Self
doesn't use associations but instead implements dictionaries with two
parallel arrays.

-- Jecel