public function rollback()
	{
		try
		{
			if( --$this->_txnumber === 0 )
			{
				if( $this->_connection->rollBack() === false ) {
					throw new MW_DB_Exception( 'Failed to roll back transaction' );
				}
			}
		}
		catch( PDOException $e )
		{
			throw new MW_DB_Exception( $e->getMessage(), $e->getCode(), $e->errorInfo );
		}
	}