using format to write to a string

[Guy Footring <Guy@AVON.sltd.dialnet.symbolics.com>]

    Date: Fri, 26 Jun 1992 10:17-0000
    From: starnet!bass!lakin@apple.com (Fred Lakin)

    Seems to work for me on a 36xx [as well as in unix CL's], 
    but not on an XL400.

    Am I doing something wrong here?

Nothing as regards CL.  Try not specifying :adjustable T on the XL (all arrays are adjustable
on the Symbolics anyway) - e.g. 

(defun test-io-to-string (&aux keep-string)
  (setf keep-string (make-array 0 :element-type 'STRING-CHAR
				:fill-pointer 0 #-Imach :adjustable #-Imach T))
  (format keep-string "Here we see ~A happen " 'BIG-BLOWUP)
  keep-string)