public function asString()
    {
        if ($this->isLeaf()) {
            $return = $this->_filter;
        } else {
            $return = '';
            foreach ($this->_subfilters as $filter) {
                $return = $return.$filter->asString();
            }
            $return = '(' . $this->_match . $return . ')';
        }
        return $return;
    }