$(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>');
});