private function _getMapValueHandler() {
        return function($key, $value) {
            foreach($this->customArgumentHandler as $name => $callable) {
                if(is_callable($callable)) {
                    $value = $callable($key, $value);
                } else
                    trigger_error("Custom argument handler $name is not callable", E_USER_WARNING);
            }
            return $value;
        };
    }