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I vote with the structure sharers on whether APPEND should copy everything.
The dangers of structure sharing are oversold, especially since APPEND is
supposed to be used for expressions that are never to be RPLAC'd.  As far
as the "cheap memory" argument goes, what structure sharing saves is
garbage collection, not storage.  (Granted, in T this is like rearranging
the deck chairs on the General Belgrano.)

I would argue for the abolition of the (APPEND l NIL) idiom for
copying l.  There should be a top-level copy primitive. (I don't see
one in my T manual.)  The only commitment APPEND should really make
is that it won't RPLAC anything.  No one should depend on its copying
lists followed by ()s, or on its not copying them.