编者按：这篇文章转载自《信息学概览》，原文是用德语写成。
Editor's Note: This article, translated from German, originally appeared in Spektrum.
编辑按：这篇文章转载自esther van der Wal的博客。
Editor's Note: This is a guest post from Esther van der wal.
乔·斯卡伯勒：在2003年9月10号，事实上我已经把整篇文章转载在了我的书里。
Joe Scarborough: - on September 10th, 2003. In fact, I've reprinted the whole thing in my book.
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