docs: function getEntity(urls, entityId) {
    // Add xhtml11.dtd to prevent missing entity errors with XHTML files
    urls.push("resource:///res/dtd/xhtml11.dtd");
  
    // Build a string of external entities
    var extEntities = "";
    for (i = 0; i < urls.length; i++) {
      extEntities += '<!ENTITY % dtd' + i + ' SYSTEM "' +
                     urls[i] + '">%dtd' + i + ';';
    }
  
    var parser = Cc["@mozilla.org/xmlextras/domparser;1"]
                    .createInstance(Ci.nsIDOMParser);
    var header = '<?xml version="1.0"?><!DOCTYPE elem [' + extEntities + ']>';
    var elem = '<elem id="elementID">&' + entityId + ';</elem>';
    var doc = parser.parseFromString(header + elem, 'text/xml');
    var elemNode = doc.querySelector('elem[id="elementID"]');
  
    if (elemNode == null)
      throw new Error(arguments.callee.name + ": Unknown entity - " + entityId);
  
    return elemNode.textContent;
  }