define-modify-macro bug?

Is this a bug in Corman Lisp 2.5? This example is very similar to one 
given in the Hyperspec, and the definition of appendf also exists in the 
version of ASDF that I am trying to use:

?(define-modify-macro appendf (&rest args) 
    append "Append onto list")
;;; Warning: Symbol &REST assumed special
APPENDF
?(setq x '(a b c))
(A B C)
?(appendf x '(d e f) '(1 2 3))
;;; An error occurred in function MACROEXPAND:
;;; Error: The variable &REST is unbound
;;; Entering Corman Lisp debug loop. 
;;; Use :C followed by an option to exit. Type :HELP for help.
;;; Restart options:
;;; 1   Abort to top level.
:c 1
;;; Returning to top level loop.
?


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