The same as a Set, except that items are compared using #== instead of #=. Almost any class named IdentityFoo is the same as Foo except for the way items are compared. In Foo, #= is used, while in IdentityFoo, #== is used. That is, identity collections will treat items as the same only if they have the same identity. For example, note that copies of a string are equal: ('abc' copy) = ('abc' copy) but they are not identitcal: ('abc' copy) == ('abc' copy) A regular Set will only include equal objects once: | aSet | aSet := Set new. aSet add: 'abc' copy. aSet add: 'abc' copy. aSet An IdentitySet will include multiple equal objects if they are not identical: | aSet | aSet := IdentitySet new. aSet add: 'abc' copy. aSet add: 'abc' copy. aSet