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> As long as we're still talking about weak pointers and related topics,
> I should note that there seem to be two very different broad categories
> of weak pointer usage.  (If you know a third, please speak up.)
> 1. Hash Consing
> This kind of usage has been mentioned on this mailing list already.
> The idea is that you need to add a slot to an object, but you don't
> have control over the object's memory layout.

> 2. Back Pointers
> One thing that would help here is a "weak set" construct,
> You can build a weak set out of a (:TEMPORARY T :TEST 'EQ) hash
> table by storing the back pointers as keys.  The values stored
> on the keys are immaterial; T would do fine! 

I did mention this in one of the earlier messages because I wanted
T's weak sets to count as part of "current practice".  Since 
an implementation using hash tables does waste 1/2 of each entry,
you're type suggestion seems reasonable.  (Quick look at arrays
in CLtL...)  Indeed, array element types are also specified by a
keyword parameter so there is precedent for that approach.

There should probably be a type parameter for the key as well.
So we'd have :KEY-TYPE and :VALUE-TYPE.

-- Jeff