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[no subject]

If this is really still subject to interpretation, I encourage the 
shared-structure approach.

For the simple cases, it would be best not to scare people away from
using these set operations because they believe them to be too
inefficient.  Odds are you wouldn't feel as bad about the possible 
redundant consing in (COPYLIST (UNION X Y)) if you need a full copy 
as you would feel about the unnecessary consing in (UNION X Y) if you 
didn't want a full copy.

In general, I think the burden should be on those who want to do 
side-effects to assure that they have taken appropriate safety 

Also, writing (COPYLIST (UNION X Y)) makes an important assumption 
explicit in the code. If (UNION X Y) is always copying, it is harder to
tell from inspection when the programmer plans to take advantage of
the fact.