Issue: UNWIND-PROTECT-CLEANUP-NON-LOCAL-EXIT (Version 4)

[gls@Think.COM]

   Date: Tue, 24 May 88 03:00:59 PDT
   From: Jon L White <edsel!jonl@labrea.stanford.edu>
   ...
   What's not OK is to block a "committed" THROW.  Without this restriction,
   for example, an "abort" to top-level while runnining the application evaluator
   noted below would never succeed; the THROW to FOO from within the cleanup
   form would serve as an effect barrier such that no throw can go past it.

	 (loop
	   (catch 'foo
	     (unwind-protect 
		   (Run-application-evaluator)
	       (throw 'foo t))))
   ...

But the very fact that you can run an arbitrary computation implies that
you can effectively block a THROW.  For example, I can rewrite the loop
shown above as:

	(labels ((again () (unwind-protect
				(Run-application-evaluator)
				(again))))
	  (again))

Admittedly this is likely to cause net stack growth in any implementation
I can think of, but semantically it is equivalent.

Another point: what good is signalling an error?  It will give handlers a
chance to run, but is the error allowed to break out without invoking the
cleanup forms anyway?

--Guy