code: function () {
                      var value;
  
                      if (input.charAt(i) !== 'u' || !$(/^url\(/)) return;
                      value = $(this.entities.quoted)  || $(this.entities.variable) ||
                              $(/^(?:(?:\\[\(\)'"])|[^\(\)'"])+/) || "";
  
                      expect(')');
  
                      return new(tree.URL)((value.value != null || value instanceof tree.Variable)
                                          ? value : new(tree.Anonymous)(value), imports.paths);
                  }
docs: Parse url() tokens  We use a specific rule for urls, because they don't really behave like standard function calls. The difference is that the argument doesn't have to be enclosed within a string, so it can't be parsed as an Expression.