iwmc-class-p bug again

[kanderso@WILMA.BBN.COM]

I think we patched this once, and someone else did too, and his patch
got accepted:

Patch to 3600-low.lisp
#||
Received-from-host: GEFILTE
RSHAPIRO@GEFILTE.BBN.COM 4/11/88 13:07:31
There's a serious bug with IWMC-CLASS-P which unfortunately
send ZMACS into the debugger in certain circumstances, and
thus needs to be fixed more or less immediately. The problem
is its call to SCL:NAMED-STRUCTURE-SYMBOL (see file >pcl>new>3600-low).
If I have presentation actions on a clos class in context T
(that is, they can happen anytime at all), and I'm in ZMACS,
and I point the mouse at a string (or presumably any other array)
I get thrown into the debugger when SCL:NAMED-STRUCTURE-SYMBOL
gets called with the string. If SCL:NAMED-STRUCTURE-SYMBOL is
replaced by SCL:NAMED-STRUCTURE-P it should work (this is not
a predicate, as the name suggests: it returns the structure
name iff the object is indeed a named structure), but
unfortunately IWMC-CLASS-P is a subst and so references to
it need to be recompiled as well....
||#
(scl:defsubst iwmc-class-p (x)
  (and (arrayp x)
       (eq (scl:named-structure-p x) 'iwmc-class)))