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The problem with the current definition is that it depends upon the
implementation.  Rather than kludging up the definition more so that
it properly describes hairier implementations of SETF, how about
simply saying that SETF (APPLY must work for AREF, BITS, and SBITS,
if those are the only useful cases.  You might also say that it
is suggested that the implementation be such that this construct
should work for as many additional cases as possible.  But I would
much rather have a specific list of those cases where it is supposed
to work than a description whose interpretation is implementation-
dependent.  Currently, I can claim that I have implemented 
SETF (APPLY correctly, since in my implementation there is no case
where the precondition would apply.  I recently added special-case
code to handle AREF, and if people agree that BITS and SBITS are
also useful, I will add them too.  I will use a strategy similar
to VanRoggen's suggested APPLY-BUT-LAST.