Chris Bitmead uid
Wed, 21 May 1997 12:51:58 +1000

>I don't see why it must be deferred to an application level.  It's a 
>problem in directed graphs.  If an object-A refers to objects that change,
>then a new version of object-A has to be created.  

That might work in theory, but in practice that could and would very
easily result in the changing of one cons cell to propogate 2 Gig of
new versioned objects just because they directly or indirectly refer
to it. Is that a good default?

Besides which Lisp doesn't maintain back-pointers so that you would
know which objects point to the one that has changed, so you would
have to do an exhaustive search of your hard disk just to change one