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From: Richard Stallman <rms@gnu.org>
Newsgroups: gmane.emacs.devel
Subject: Re: Using empty_string as the only "" string
Date: Tue, 24 Apr 2007 22:05:14 -0400
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Xref: news.gmane.org gmane.emacs.devel:70040
Archived-At: <http://permalink.gmane.org/gmane.emacs.devel/70040>

    ELISP> (eq (substring "ab" 0) (substring "ab" 0))
    nil
    ELISP> (eq (substring "ab" 1) (substring "ab" 1))
    nil
    ELISP> (eq (substring "ab" 2) (substring "ab" 2))
    t

    That doesn't seem a nice behaviour.

Any program that _depends_ on eq to return nil in the last case 
is taking a risk.