All arrays can be adjustable?

["Gail Zacharias" <GZ%OZ.AI.MIT.EDU@XX.LCS.MIT.EDU>]

The description of ADJUST-ARRAY (bottom of p. 297) says "It is not permitted
to call adjust-array on an array that was not created with the :adjustable
option."

Of course, assuming "not permitted" means "is an error" rather than "signals
an error", an implementation is free to make an extension to CL and have
ADJUST-ARRAY work on some arrays which weren't created with the :ADJUSTABLE
option, and non-portable code is free to take advantage of such an extension.
It seems rather pointless to me, however.  Unless the user has detailed
knowledge of exactly which combinations of other arguments to MAKE-ARRAY will
allow ADJUST-ARRAY to work, he has no business trying to adjust an array which
he didn't ask to be adjustable.  A friendly implementation would signal an
error, at least when running in a safe mode.
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