Union and friends

[Scott E. Fahlman <Fahlman@CMU-CS-C.ARPA>]

I agree with KMP that Union and friends would be much more useful if
shared substructure is allowed.  Usually you want the efficiency and
don't care about the purity of the list, and if you do care you can copy
the list with little loss of efficiency.  (Note that NUNION and friends
are still different, in that they allow the arguments to be clobbered
and not merely shared.)  Despite this preference, I note that whoever
implemented our version of UNION interpreted the manual as requiring a
fresh copy.  In any event, a clarification is needed.

-- Scott