public function toString( array $types, array $translations = array(), array $plugins = array() )
	{
		$query = new Zend_Search_Lucene_Search_Query_Boolean();

		foreach( $this->_expressions as $expr )
		{
			if( ( $itemstr = $expr->toString( $types, $translations, $plugins ) ) !== '' ) {
				$query->addSubquery( $itemstr, self::$_operators[$this->_operator] );
			}
		}

		return $query;
	}