Add Calil link on Amazon page

amazon_calil.user.js

// ==UserScript==
// @name           Amazon Calil
// @namespace      http://gist.github.com/2156262
// @description    Show calil page from Amazon book listing
// @include        http://*.amazon.*
// ==/UserScript==

var DEBUG=true;
showCalil();
function showCalil() {
  var val = document.getElementById("ASIN").value;
  if (val != '') {


    var header = document.evaluate("//div[@class='buying']/a", document, null, XPathResult.FIRST_ORDERED_NODE_TYPE, null).singleNodeValue;
    if (header == undefined) {
      header = document.evaluate("//div[@class='buying']//a[contains(@id, 'contribut')]", document, null, XPathResult.FIRST_ORDERED_NODE_TYPE, null).singleNodeValue;
    }

    if (header) {
      var link = document.createElement('a');
      link.setAttribute('href', 'http://calil.jp/book/' + val);
      link.setAttribute('target', '_blank');
      link.innerHTML = '<span style=\"font-size:90%; background-color:#ffffcc;\">&raquo;Open Calil<br></span>';
      header.parentNode.insertBefore(link, header);
    }
  }
}

function debug() {
  if (typeof DEBUG != 'undefined' && DEBUG) {
    console.log.apply(this, arguments);
  }
}