Dear Richard,

Oops! I wrote:

Richard:
> > IdentitySet is best regarded as a special case of PluggableSet
> 

Bill:
> I beg your pardon? OK, I think I see your point, if you consider
> identity to be a kind of equality. But the point of IdentitySet, it
> seems to me, is that identity is *more general* than equality. Identity
> entails equality, but not the other way around. I know that heredity in
> Smalltalk is based on behavior, but logically, Identity should be a
> superclass of Set (and the differences in behavior are not that great).
> 

I said the opposite of what I meant. Identity is a more specific
requirement than equality. <blush> But that does make IdentitySets more
general than EqualitySets (sic). 

And, upon reflection, I think I understand your point about
PluggableSets better. Thank you. :-) But PluggableSets do not seem more
general than Sets.

Best,

Bill