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From: Richard M Stallman <rms@gnu.org>
Newsgroups: gmane.emacs.devel
Subject: Re: Two binding features
Date: Thu, 08 May 2008 18:28:20 -0400
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progv in CL is implemented using unwind-protect.
That does the job, but maybe it would be better
to implement it as a primitive, making real bindings.

The current implementation has one advantage: it uses
just one specpdl slot for any number of variables.
But I suspect that the unwind-protect implementation
will give wrong results in some cases, for instance
when buffer-local bindings are in use for the same
variables.