protected function _commentCB($m)
    {
        $hasSurroundingWs = (trim($m[0]) !== $m[1]);
        $m = $m[1]; 
        // $m is the comment content w/o the surrounding tokens, 
        // but the return value will replace the entire comment.
        if ($m === 'keep') {
            return '/**/';
        }
        if ($m === '" "') {
            // component of http://tantek.com/CSS/Examples/midpass.html
            return '/*" "*/';
        }
        if (preg_match('@";\\}\\s*\\}/\\*\\s+@', $m)) {
            // component of http://tantek.com/CSS/Examples/midpass.html
            return '/*";}}/* */';
        }
        if ($this->_inHack) {
            // inversion: feeding only to one browser
            if (preg_match('@
                    ^/               # comment started like /*/
                    \\s*
                    (\\S[\\s\\S]+?)  # has at least some non-ws content
                    \\s*
                    /\\*             # ends like /*/ or /**/
                @x', $m, $n)) {
                // end hack mode after this comment, but preserve the hack and comment content
                $this->_inHack = false;
                return "/*/{$n[1]}/**/";
            }
        }
        if (substr($m, -1) === '\\') { // comment ends like \*/
            // begin hack mode and preserve hack
            $this->_inHack = true;
            return '/*\\*/';
        }
        if ($m !== '' && $m[0] === '/') { // comment looks like /*/ foo */
            // begin hack mode and preserve hack
            $this->_inHack = true;
            return '/*/*/';
        }
        if ($this->_inHack) {
            // a regular comment ends hack mode but should be preserved
            $this->_inHack = false;
            return '/**/';
        }
        // Issue 107: if there's any surrounding whitespace, it may be important, so 
        // replace the comment with a single space
        return $hasSurroundingWs // remove all other comments
            ? ' '
            : '';
    }