$(document).ready(function () {
  'use strict';

  // Use attr() to add an id, rel, and title.
  $('div.chapter a[href*="wikipedia"]').attr({
    rel: 'external',
    title: function () {
      return 'Learn more about ' + $(this).text() + ' at Wikipedia.';
    },
    id: function (index, oldValue) {
      return 'wikilink-' + index;
    }
  });

  // Add "back to top" links.
  $('<a href="#top">back to top</a>').insertAfter('div.chapter p');
  $('<a id="top"></a>').prependTo('body');
  // insertBefore <elem> prependTo <children /> appendTo </elem> insertAfter

  // Create footnotes.
  var $notes = $('<ol id="notes"></ol>').insertBefore('#footer');
  $('span.footnote').each (function (index) {
    $(this)
      .before([
        '<a href="#footnote-',
        index + 1,
        '" id="context-',
        index + 1,
        '" class="context">',
        '<sup>',
        index + 1,
        '</sup></a>'
      ].join(''))
      .appendTo($notes)
      .wrap('<li id="footnote-' + (index + 1) + '"></li>');
  });
});