function _get_zs($i, $j = -1) {
        if ($j == -1) $j = $i;
        return substr($this->_txt, $this->_zmap[$i]['start'], $this->_zmap[$j]['end'] - $this->_zmap[$i]['start']);
    }
function _get_zs($i, $j = -1) {
        if ($j == -1) $j = $i;
        return substr($this->_txt, $this->_zmap[$i]['start'], $this->_zmap[$j]['end'] - $this->_zmap[$i]['start']);
    }
function _get_zs ( $ i , $ j = - 1 ) { if ( $ j == - 1 ) $ j = $ i ; return substr ( $ this -> _txt , $ this -> _zmap [ $ i ] [ 'start' ] , $ this -> _zmap [ $ j ] [ 'end' ] - $ this -> _zmap [ $ i ] [ 'start' ] ) ; }
get one z by ZMAP
get one z by ZMAP