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<!DOCTYPE html> <html lang="en" class="no-js"> <head>  <meta charset="utf-8"> <title>Wikibooks</title> <script> document.documentElement.className = document.documentElement.className.replace( /(^|\s)no-js(\s|$)/, "$1js-enabled$2" ); </script> <meta name="viewport" content="initial-scale=1,user-scalable=yes"> <link rel="apple-touch-icon" href="/static/apple-touch/wikibooks.png"> <link rel="shortcut icon" href="/static/favicon/wikibooks.ico"> <link rel="license" href="//creativecommons.org/licenses/by-sa/4.0/"> <style> .sprite{background-image:linear-gradient(transparent,transparent),url(portal/wikibooks.org/assets/img/sprite-e35f24c8.svg);background-repeat:no-repeat;display:inline-block;vertical-align:middle}.svg-Commons-logo_sister{background-position:0 0;width:47px;height:47px}.svg-MediaWiki-logo_sister{background-position:0 -47px;width:42px;height:42px}.svg-Meta-Wiki-logo_sister{background-position:0 -89px;width:37px;height:37px}.svg-Wikibooks-logo_sister{background-position:0 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