public function registerCombinator($name, $object)
    {
        if (is_callable($object)) {
            $this->_combinators[$name] = array(
                "classname" => "CssParserCombinatorUserDefined",
                "user_def_function" => $object
            );
        } else {
            $this->_combinators[$name] = array(
                "classname" => $object,
                "user_def_function" => null
            );
        }
    }