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From: Richard Stallman <rms@gnu.org>
Newsgroups: gmane.emacs.devel
Subject: Re: GC: marking traversal and pure symbols
Date: Sat, 07 Jul 2007 09:06:16 -0400
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To: David Kastrup <dak@gnu.org>
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Xref: news.gmane.org gmane.emacs.devel:74432
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    Would it not make sense to restrict the purity notion to the value
    cell of a symbol?

On some platforms, pure storage is (or at least was) stored in
read-only memory.  If nil is in read-only memory, we cannot make
its plist cell writable.