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Another (major) difference

   Redistributed: CommonLoops.pa
   From: larus@paris.Berkeley.EDU (James Larus)
   Reply-To: larus@ginger.Berkeley.EDU
   Date: Mon, 29 Aug 88 09:52:07 PDT


	   A major difference between the AAAI release of PCL and
   previous releases of PCL is minimally documented in the release notes.
   PCL now checks whether slots are "unbound", in other words, you can no
   longer assume that fields within an object are initialized to NIL.
   This is a major change to the sematics of PCL (and a major difference
   from the semantics of CL structures).

I beg to differ.  On p.308 of the Silver Bible regarding DEFSTRUCT:

"If no {\it default-init} is specified, then the initial contents of
the slot are undefined and implementation-dependent."

It is only by convention, not by law, that existing Common Lisp
implementations initialize unspecified DEFSTRUCT slots to NIL.