docs: function domain(url) {
    var cookie = exports.cookie;
    var levels = exports.levels(url);
  
    // Lookup the real top level one.
    for (var i = 0; i < levels.length; ++i) {
      var cname = '__tld__';
      var domain = levels[i];
      var opts = { domain: '.' + domain };
  
      cookie(cname, 1, opts);
      if (cookie(cname)) {
        cookie(cname, null, opts);
        return domain;
      }
    }
  
    return '';
  }