protected function getNextTag()
    {
        $tagLetter = substr( $this->currentTag, 0, 1 );
        $tagNumber = intval( substr( $this->currentTag, 1 ) );
        $tagNumber++;
        if ( $tagLetter == 'Z' && $tagNumber == 10000 )
        {
            $tagLetter = 'A';
            $tagNumber = 1;
        }
        if ( $tagNumber == 10000 )
        {
            $tagLetter++;
            $tagNumber = 0;
        }
        $this->currentTag = $tagLetter . sprintf( "%04s", $tagNumber );
        return $this->currentTag;
    }