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Circular structures [was Re: IEEE 754 floating-point arithmetic is not completely ordered]

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On Apr 16, 2005, at 10:53 AM, Jens Axel Søgaard wrote:

> Bradley Lucier wrote:
>> Well, it depends on what your goal is. ...
> Here are some potential goals:
> ...
> 3) default-compare should define a total order on almost all Scheme 
> values

What is "almost all"?  How do a and b compare in the following?

[descartes:~/programs/folding/2] lucier% gsc
loading /usr/local/Gambit-C/gambcext.scm
Gambit Version 4.0 beta 12

 > (define a (cons #f #f))
 > (set-car! a a)
 > (set-cdr! a a)
 > (define b (cons #f #f))
 > (set-car! b b)
 > (set-cdr! b b)
 > (equal? a b)   ;;; doesn't terminate

Is default-compare compatible with equal?