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Backquote idioms

    Date: Wednesday, 22 May 1985, 10:56-EDT
    From: Guy Steele <gls%AQUINAS@THINK.ARPA>
    By the way, ',@x is not always an error!  It does make sense if x is
    exactly a singleton list.  Sometimes this is useful in obscure cases.

    Similarly, ,@',,@x makes sense if x is a singleton list.  See the end of
    this note.

I think "is an error" is still the right phrase to apply.
Just because there happens to be a value that does not result
in an error does not make it proper.  I think it is *MUCH*
clearer to write ',(CAR X).  You can do additional error
checking on X if you like, and signal a comprehensible
error, rather than depending on QUOTE to check!  (I have
seen implementations where QUOTE did not check).