This program is described closely in "Understanding Computation"
chapter 6 by Tom Stuart. <http://computationbook.com/>
Japanese translation will be published soon.
<http://www.oreilly.co.jp/books/9784873116976/>
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47447 b2dd03c8-39d4-4d8f-98ff-823fe69b080e