Text_Diff_Renderer::_blockHeader

    function _blockHeader($xbeg, $xlen, $ybeg, $ylen)
    {
        if ($xlen > 1) {
            $xbeg .= ',' . ($xbeg + $xlen - 1);
        }
        if ($ylen > 1) {
            $ybeg .= ',' . ($ybeg + $ylen - 1);
        }

        // this matches the GNU Diff behaviour
        if ($xlen && !$ylen) {
            $ybeg--;
        } elseif (!$xlen) {
            $xbeg--;
        }

        return $xbeg . ($xlen ? ($ylen ? 'c' : 'd') : 'a') . $ybeg;
    }