>  factor out a new 
> function 'hashobject:', and override that to invoke '^object 
> identityHash', '^object hash', or 'hashfunc value: object'.)

I would say factor out two methods:  hashObject: and compare:with:
 
> IMHO, it is not good to add 'yet another special collection' 
> especially if we don't have to.

I guess I agree if it's certain that it will work, and won't impact
performance.

- Stephen